Thunderbird is apersonal information manager (PIM) with e-mail,newsgroup, news feed, andinstant messaging functionality, with calendar functionality inbuilt since version 78.0 and previously available from theLightning calendar extension. Additional features are available fromextensions, which allow the addition of features through the installation ofXPInstall modules (known as "XPI" or "zippy" installation) via the add-ons Web site that also features an update functionality to update the extensions.[citation needed]
Thunderbird supports a variety ofthemes for changing its overall look and feel. These packages ofCSS andimage files can be downloaded via the add-ons website at Mozilla Add-ons.[12]
On 13 June 2022, it was announced that the Android appK-9 Mail had been taken over byMZLA Technologies Corporation, a subsidiary of theMozilla Foundation. The plan was for K-9 Mail to be rebranded as Thunderbird for Android, including sync with Thunderbird on PC, integrating Thunderbird's automated account setup system, message filtering, and improvements to folders.[14]
On 30 October 2024, the first stable release of Thunderbird for Android (Thunderbird Mobile) was launched as version 8.0.[15] On 6 May 2025, the stable version 10 and the beta of version 11 were released.[16] Thunderbird for Android can be installed viaGoogle Play andF-Droid, among others.[17]
Thunderbird manages multiple email, newsgroup, and news feed accounts and supports multiple identities within accounts. Features such as quick search, saved search folders ("virtual folders"), advanced messagefiltering, message grouping, and tags help manage and find messages. OnLinux-based systems, system mail (movemail) accounts were supported until version 91.0.[18] Thunderbird provides basic support for system-specific new email notifications and can be extended with advanced notification support using an add-on.[19]
Thunderbird incorporates aBayesianspam filter, awhitelist based on the included address book, and can also understand classifications by server-based filters such asSpamAssassin.[20]
IMAP. Thunderbird has implemented many of the capabilities in IMAP, in addition to adding their own extensions and the de facto standards by Google and Apple.[21]
Email providers have increasingly adoptedOAuth authentication in addition, or instead of, more traditional authentication methods, most notably byGmail andOutlook. Thunderbird has full support for OAuth authentication and provides instructions for Outlook users transitioning from Outlook's now-deprecated "basic authentication".[24]
Thunderbird provides mailbox format support usingplugins, but this feature is not yet enabled due to related work in progress.[25] The mailbox formats supported as of July 2014[update] are:
mbox – Unix mailbox format (one file holding many emails)
maildir – known as maildir-lite (one file per email). As of August 2019[update] "there are still many bugs", so this is disabled by default.[26]
Thunderbird also usesMork and (since version 3) MozStorage (which is based onSQLite) for its internal database. Mork was due to be replaced with MozStorage in Thunderbird 3.0,[27] but the 8.0 release still uses the Mork file format.
Since version 13, Thunderbird has integrated support for uploading files to online file storage services. Users can link large files instead of attaching them directly to the email message. Supported providers includedUbuntu One andDropbox.[28] Currently, file linking can be used throughadd-ons or by connecting to aWebDAV provider.[29]
Thunderbird provides awysiwyg editor for composing messages formatted with HTML (default). The delivery format auto-detect feature will send unformatted messages as plain text (controlled by a user preference). Certain special formatting likesubscript, superscript andstrikethrough is available from the Format menu. The Insert >HTML menu provides the ability to edit theHTML source code of the message. There is basic support for HTML template messages, which are stored in a dedicated templates folder for each account.
Mozilla Thunderbird calendar screen
Markdown support is provided through the Markdown Here Revival add-on.[30]
Thunderbird provides security features such asTLS/SSL connections toIMAP andSMTP servers. It also offers inbuilt support for secure email withdigital signing and messageencryption throughOpenPGP (using public and privatekeys) orS/MIME (usingcertificates). Any of these security features can take advantage ofsmartcards with the installation of additional extensions.
Other security features may be added through extensions. Up to version 68, theEnigmail extension was required forOpenPGP support (now inbuilt).
Optional security protections also include disabling loading of remote images within messages, enabling only specific media types (sanitizer), and disablingJavaScript.
TheFrench military uses Thunderbird and contributes to its security features, which are claimed to match the requirements forNATO's closed messaging system.[31]
As with any software, there may be limitations to the number and sizes of files and objects represented. For example,POP3 folders are subject tofilesystem design limitations, such as maximum file sizes on filesystems that do not havelarge-file support, as well as possible limitations oflong filenames, and other issues.[32]
While primarily used as an email client, Thunderbird is also anews aggregator, which is namedNews Feeds and Blogs in the application. It supportsRSS and more recently addedAtom as well.[33]
Thunderbird is also anewsreader forUsenet, using theNNTP protocol and supports NNTPS. Intended for reading use, it does not supportNZB for binary downloading, nor does it have a search function for the newsgroups.[36]
As of version 91, Thunderbird introduced a built-inPDF document reader, allowing users to open and view PDF attachments or files directly within the application without needing an externalPDF viewer.[37]
In June 2022, the Thunderbird project announced that it would be collaborating with email clientK-9 for Android to build a Thunderbird version for Android.[43] To this end, K-9 lead developer Christian Ketterer joined the Thunderbird team in 2022. The plan was to transformK-9 Mail into Thunderbird on Android, including the respective name change and adoption of the Thunderbird branding. As soon as K-9 Mail had been brought into alignment with Thunderbird's feature set and visual appearance, this changeover would take place, according to Thunderbird's Jason Evangelho.
In the 2022 Financial Report published on May 8, 2023, Ryan Sipes (Product and Business Development Manager) mentions plans of hiring an iOS developer later that same year to start laying the foundation for Thunderbird on iOS.[44]
On 30 October 2024, the first stable release of Thunderbird for Android (Thunderbird Mobile) was launched as version 8.0.[15] Thunderbird for Android can be installed viaGoogle Play andF-Droid, among others.[17] The Thunderbird team had planned in 2022 to maintain K-9 for one year after the release of Thunderbird Mobile, i.e. until roughly October 2025.[45]
On 6 May 2025, the stable version 10 of Thunderbird Mobile and the beta of version 11 were released.[16]
As of 2025, an initial version of Thunderbird's iOS client was available via Apple'sTestFlight.[46]
Logo of Thunderbird 2004–2009Logo of Thunderbird 2009–2018
Originally launched asMinotaur shortly afterPhoenix (the original name for Mozilla Firefox), the project failed to gain momentum. With the success of Firefox, however, demand increased for a mail client to go with it, and the work on Minotaur was revived under the new name of Thunderbird, and migrated to the new toolkit developed by the Firefox team.
A pre-release version of Thunderbird
On December 7, 2004, version 1.0 was released, and received more than 500,000 downloads in its first three days of release, and 1,000,000 in ten days.[49][50]
Significant work on Thunderbird restarted with the announcement that from version 1.5 onward the main Mozilla suite would be designed around separate applications using this new toolkit. This contrasts with the previous all-in-one approach, allowing users to mix and match the Mozilla applications with alternatives. The original Mozilla Suite continues to be developed asSeaMonkey.
On December 23, 2004,Project Lightning was announced which tightly integratedcalendar functionality (scheduling, tasks, etc.) into Thunderbird. Lightning supports the full range of calendar mechanisms and protocols supported by the Mozilla Calendar infrastructure, just as with modern (post-0.2) Sunbird.
On October 11, 2006,Qualcomm and the Mozilla Foundation announced that "future versions ofEudora will be based upon the same technology platform as the open source Mozilla Thunderbird email program."[51] The project was code-namedPenelope.
In late 2006,Debian rebranded Thunderbird asIcedove due to trademark and copyright reasons. This was the second product to be rebranded.[10][52]
On July 26, 2007, the Mozilla Foundation announced that Thunderbird would be developed by an independent organization, because theMozilla Corporation (a subsidiary of the foundation) was focusing on Mozilla Firefox development.[53]
On September 17, 2007, the Mozilla Foundation announced the funding of a new internet communications initiative with David Ascher ofActiveState. The purpose of this initiative was "to develop Internet communications software based on the Thunderbird product, code, and brand".[54]
On February 19, 2008,Mozilla Messaging started operations as a subsidiary of the Mozilla Foundation responsible for the development of email and similar communications. Its initial focus was on the then upcoming version of Thunderbird 3. Alpha Preview releases of Thunderbird 3 were codenamed "Shredder".
Thunderbird 5.0, the first version under the software's rapid release cycle
On April 4, 2011, Mozilla Messaging was merged into theMozilla Labs group of the Mozilla Foundation.[55]
Following in the footsteps of Firefox, Thunderbird switched to a rapid release cycle in 2011.[56] Version 4.0 was skipped so as to re-align Thunderbird with Firefox's version 5.0, both released in June 2011.[57] By the end of the year it had reached version 9.0.[58]
On July 6, 2012, a confidential memo from Jb Piacentino, the Thunderbird Managing Director at Mozilla, was leaked and published toTechCrunch.[59] The memo indicated that Mozilla would be moving some of the team off the project and further development of new features would be left up to the community. The memo was slated for release on July 9, 2012. A subsequent article by the Executive Chair of Mozilla,Mitchell Baker, stated Mozilla's decision to make a transition of Thunderbird to a new release and governance model.[60][61] On July 6, 2012, Mozilla announced the company was dropping the priority of Thunderbird development because the continuous effort to extend Thunderbird's feature set was mostly fruitless. The new development model shifted to Mozilla offering only "Extended Support Releases", which deliver security and maintenance updates, while allowing the community to take over the development of new features.
On November 25, 2014, Kent James of the volunteer-led Thunderbird Council announced on the Thunderbird blog that active contributors to Thunderbird gathered at the Mozilla office in Toronto and discussed the future of the application. They decided that more staff were required working full-time on Thunderbird so that the Thunderbird Team could release a stable and reliable product and make progress on features that had been frequently requested by the community.[62]
On December 1, 2015, Mozilla Executive ChairMitchell Baker announced in a company-wide memo that Thunderbird development needed to be uncoupled from Firefox.[63] She referred to Thunderbird developers spending large efforts responding to changes to Mozilla technologies, while Firefox was paying a tax to support Thunderbird development. She also said that she does not believe Thunderbird has the potential for "industry-wide impact" that Firefox does.[64] Mozilla remained interested in having a role in Thunderbird, but sought more assistance to help with development.[65] Therefore, at the same time, it was announced that Mozilla Foundation would provide at least a temporary legal and financial home for the Thunderbird project.[66]
Logo of Thunderbird 2018–2023Logo of Thunderbird since 2023[update]
On May 9, 2017, Philipp Kewisch announced that the Mozilla Foundation would continue to serve as the legal and fiscal home for the Thunderbird project, but that Thunderbird would migrate off Mozilla Corporation infrastructure, separating the operational aspects of the project.[67] Mozilla brought Thunderbird back in-house in an announcement on May 9, 2017, and continued to support its development.[68][69] The Thunderbird development team expanded by adding several new members and overhauled security and the user interface.[70]
The interim/beta versions Thunderbird 57 and 58, released in late 2017, began to make changes influenced byFirefox Quantum, including a new "Photon" user interface.[71][72] Despite the removal in Firefox Quantum of support forXUL-basedlegacy add-ons in favor ofWebExtensions, the stable/ESR release of Thunderbird 60 in mid-2018 continued to support them, although most would require updates, and it did not support WebExtensions except for Themes.[73][74] In 2018, work was underway for planned support in Thunderbird 63 of WebExtensions and to continue to "somewhat" support legacy addons, according to Mozilla.[75]
With the release of Thunderbird 68 in August 2019 it now only supports WebExtension addons.[76] Legacy Addons can still be used if a special "legacy mode" is enabled, but even for this, the legacy Addon has to be converted.[77] Alongside the transition,OpenPGP support was integrated directly into Thunderbird as a standard feature, seeking to supplant theEnigmail extension.[78] Mainly for licensing reasons, this is no longer based on GnuPG, but on theRNP library, which has more liberal licensing terms.[79]
On January 28, 2020, the Mozilla Foundation announced that the project would henceforth be operating from a new wholly owned subsidiary, MZLA Technologies Corporation, in order to explore offering products and services that were not previously possible and to collect revenue through partnerships and non-charitable donations.[80][81]
As of version 78.7.1, Thunderbird will no longer allow installation of addons that use Legacy WebExtensions. Only MailExtensions are now compatible with Thunderbird.[82][83] MailExtensions are WebExtensions but with "some added features specific to Thunderbird".[84]
Thunderbird 91 features various UI improvements (including a new account setup workflow),Apple silicon support,CardDAV address book support, built-in import and export tools for Thunderbird profiles, thePDF.jsPDF viewer, and the ability to encrypt emails to BCC recipients.[85][86][87][88]
On June 13, 2022, it was announced that the Mozilla Thunderbird team would take over development of theAndroid email client appK-9 Mail, with plans for it to eventually become a mobile version of Thunderbird with synchronisation support.[14]
In November 2024, Thunderbird Mobile became available for download,[89] and in December 2024,iodéOS, an open sourceAndroidoperating system, began including Thunderbird Mobile as the default email client for some 30+ devices.[90]
Thunderbird development releases occur in three stages, called Beta, Earlybird, and Daily, which correspond to Firefox's Beta, Aurora, and Nightly stages. The release dates andGecko versions are exactly the same as Firefox; for example, Firefox 7 and Thunderbird 7 were both released on September 27, 2011, and were both based on Gecko 7.0.
On April 4, 2025, the Thunderbird team announced a future suite of paid,cloud-based services under the name "Thunderbird Pro". The announcement positioned the new services as an open-source, privacy-focused ecosystem to compete with integrated platforms likeGoogle Workspace andMicrosoft 365. The team clarified that the core Thunderbird email application will remain free of charge and that the Pro services are an optional, separate offering designed to provide features not possible in a client-only application, such as server-side processing and storage.
The stated business model is a subscription-based service to cover infrastructure costs, with plans to introduce a limited free tier once the service is sustainable. The announcement emphasized that all components would be open source and built on open standards to avoid vendor lock-in.[91][92][93]
The services announced as part of the Thunderbird Pro suite include:
Thundermail: An email service, including calendar and contact hosting, intended to provide an ad-free and privacy-respecting alternative to existing providers. The service will offer email addresses atthundermail.com ortb.pro[94][92] as well as support for custom domains. Thundermail will be built on the open-source software stack Stalwart.[92]
Thunderbird Appointment: A scheduling tool allowing users to share a link for others to book meetings on their calendar.[95] As of June 9, 2025, the service is in closed beta.[96]
Thunderbird Send: An end-to-end encrypted file-sharing service for large files. The project is described as a successor toFirefox Send. The source code for send is published onGitHub.[97]
Thunderbird Assist: An experimental, opt-in feature to provide AI capabilities. Developed in partnership with Flower AI, it is designed to use federated learning for on-device processing where possible and confidential remote compute for privacy-preserving server-side processing.[92][98]
At the time of the announcement, Appointment was in a closed beta, Send was in alpha, and a waitlist had been opened for the Thundermail beta.[91]
^Castellani, Alessandro (February 9, 2023)."Why We're Rebuilding The Thunderbird Interface From Scratch". MZLA Technologies Corporation. RetrievedFebruary 11, 2023.Thunderbird is literally a bunch of code running on top of Firefox. All the tabs and sections you see in our applications are just browser tabs with a custom user interface.
^"Warpzilla – Mozilla for OS/2". Os2news.warpstock.org. Archived fromthe original on September 19, 2018. RetrievedAugust 6, 2018.Firefox 52esr is the last version that will be ported to OS/2.
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