This article provides information about the changes in Firefox 64 that will affect developers. Firefox 64 was released on December 11, 2018.

Changes for web developers

Developer tools

• TheAccessibility info-bar has been enhanced to include information about the color contrast ratio of text or images on the page (Firefox bug 1473037).

• Responsive Design Mode device selection is now saved between sessions (Firefox bug 1248619).

• Resources that are potentially affected byTracking Protection are now indicated in theNetwork Monitor (Firefox bug 1333994).

• TheWeb Console got improvements around entering and evaluating code:

  • Autocomplete for Console expressions is now case-insensitive (Firefox bug 672733).
  • You can now step through theConsole expression history with bash-like reverse search (Firefox bug 1024913).
  • Evaluated code in the Console is now syntax-highlighted in both the input and output (Firefox bug 1463669).

• Stepping in theJavaScript Debugger also had some improvements:

  • Stepping out of a function in the Debugger now skips the return value (Firefox bug 923975).

Removals

• The Developer Tools GCLI has been removed (Firefox bug 1429421).

HTML

No changes.

CSS

• CSS Scrollbars spec functionality has been enabled by default (Firefox bug 1492012).
• Interaction Media Features implemented, includingpointer:coarse (Firefox bug 1035774). Forany-pointer andany-hover implementation, seeFirefox bug 1483111.
• Theprefers-reduced-motion media feature is now supported in Firefox for Android (Firefox bug 1478505).
• CSS<gradient> values now support multi-position color stop syntax, e.g.,yellow 25%, yellow 50% can now be writtenyellow 25% 50% (Firefox bug 1352643).
• Thetext-transform property now accepts thefull-size-kana value (Firefox bug 1498148).
• Support added for-webkit-appearance to alleviate associated web compat issues (Firefox bug 1368555).
• Closely associated with the above update, we've also removed most of the Firefox-specific-moz-appearance values (Firefox bug 1496720).
• display:list-item is now supported on<legend> elements (Firefox bug 1486602).
• SVGpath()s, as usable inoffset-path, are now animatable (Firefox bug 1486094).
• If a selector chain or group includes a-webkit--prefixed pseudo-element, that pseudo-element no longer invalidates it (seeFirefox bug 1424106 for the details, andFirefox bug 1486325 for enabling this feature).

Removals

• The non-standard-moz-box and-moz-inline-boxdisplay values have been hidden from web content (Firefox bug 1496961).
• The non-standard::-moz-treepseudo-element has been hidden from web content (Firefox bug 1496961).
• Thelayout.css.filters.enabled preference has been removed;CSS filters can no longer be disabled (Firefox bug 1408841).
• A previous change — to swap the values of the 2-valueoverflow syntax so block is specified first and inline second (Firefox bug 1481866) — has been reverted (Firefox bug 1492567). SeeBug 1481866 comment 14 for why.

SVG

No changes.

JavaScript

• The TC39Well-formed JSON.stringify proposal has been implemented, to preventJSON.stringify from returning ill-formed Unicode strings (Firefox bug 1469021).
• Proxied functions can now be used withFunction.prototype.toString (Firefox bug 1440468).
• In theWebAssembly.Global() constructor, if no value is specified a typed 0 value is used. This is specified by theDefaultValue algorithm (Firefox bug 1490286).

APIs

DOM

• A number of changes related to theFullscreen API have been implemented:

  • The API has been unprefixed (Firefox bug 1269276).
  • TheElement.requestFullscreen() andDocument.exitFullscreen() methods both now return aPromise, which is resolved when the mode change is complete (Firefox bug 1188256 andFirefox bug 1491212).
  • Previously,fullscreenchange andfullscreenerror events were dispatched to theDocument first, then theElement. This has been reversed so that the element gets the event first. This matches the latest specification as well as the behavior of Google Chrome (Firefox bug 1375319).

• TheWebVR API (1.1) is now re-enabled in macOS (Firefox bug 1476091).

• Window.screenLeft andWindow.screenTop have been implemented, as aliases ofWindow.screenX andWindow.screenY (Firefox bug 1498860).

• TheXMLHttpRequest.getAllResponseHeaders() method now returns header names all in lowercase, as per spec (Firefox bug 1398718).

• The legacyHTMLAllCollection interface has been updated as per recentspec updates (Firefox bug 1398354).

• Navigator.buildID now returns a fixed timestamp as a privacy measure (Firefox bug 583181).

• The followingDocument.execCommand() UI feature commands have been disabled by default (Firefox bug 1490641:

  • enableObjectResizing
  • enableInlineTableEditing
  • enableAbsolutePositionEditor

Service workers

• TheServiceWorkerContainer.startMessages() method has been implemented (Firefox bug 1263734).

Media, Web Audio, and WebRTC

• ThePannerNode.refDistance,PannerNode.maxDistance,PannerNode.rolloffFactor, andPannerNode.coneOuterGain properties now correctly throw exceptions when set to values outside their accepted ranges (Firefox bug 1487963).
• RTCRtpEncodingParameters settings changed viaRTCRtpSender.setParameters() used to not update if the changes were made during a call. They will now update live, without renegotiation (Firefox bug 1253499).
• RTCIceCandidateStats.relayProtocol has been implemented — this is the standardized version ofRTCIceCandidateStats.mozLocalTransport, which has now been deprecated (Firefox bug 1435789).
• Automatic Gain Control (AGC) is now enabled by default; this can be changed using the preferencemedia.getusermedia.agc_enabled (Firefox bug 1496714).

Removals

• TheWindow.event property, added in Firefox 63 to aid with web compat issues, has been put behind a pref (dom.window.event.enabled) and disabled by default in release versions for now due to other issues that have been uncovered (Firefox bug 1493869). Note that this was actually done late in the Firefox 63 release cycle, but we are mentioning it here just in case.
• TheLocalMediaStream interface and itsstop() method have been removed (Firefox bug 1258143). This method is no longer available with the deprecation ofLocalMediaStream. See theStopping a video stream section to learn how to stop an entire stream.
• TheAudioStreamTrack andVideoStreamTrack interfaces have been removed, as both have been deprecated for some time (Firefox bug 1377146). Their functionality has been merged intoMediaStreamTrack; tracks are now identified by the value of theirkind property, such asaudio orvideo.

Security

• The Symantec CA Distrust plan has been implemented (seeFirefox bug 1409257; see also theMozilla's Plan for Symantec Roots discussion for more details).
• Referrer-Policy can now be used to govern resources fetched via stylesheets (Firefox bug 1330487) — seeIntegration with CSS for more information.

Plugins

No changes.

WebDriver conformance (Marionette)

API changes

• Deprecated command end-points without theMarionette:,L10n:, orAddon: prefix (includingsingeTap) have been removed (Firefox bug 1504478,Firefox bug 1504940).

Bug fixes

• SynthesizedShift key events by usingWebDriver:PerformActions didn't result in capitalized letters (Firefox bug 1405370).
• WebDriver:Navigate could cause an infinite hang if the tab's underlying content process is changed multiple times during that navigation (Firefox bug 1504807).
• To improve the performance, and to reduce the memory footprint of Firefox the default page to be loaded for a new tab or window is no longerabout:newtab butabout:blank (Firefox bug 1506643).
• The content blocking introduction panel, which was shown on various web pages and caused element interactions to fail is disabled by default now (Firefox bug 1488826).

Changes for add-on developers

API changes

Menus

• A new API,menus.overrideContext(), can be called from thecontextmenu DOM event to set a custom context menu in extension pages. This API allows extensions to hide all default Firefox menu items in favor of providing a custom context menu UI. This context menu can consist of multiple top-level menu items from the extension, and may optionally include tab or bookmark context menu items from other extensions. Seethis blog post for more details.

  • menus.overrideContext() was implemented in (Firefox bug 1280347).
  • TheshowDefaults: false option, which can be used to hide the default context menu options, was implemented in (Firefox bug 1367160).
  • documentURLPatterns can now be used to match amoz-extension:// document URL, even ifmenus.overrideContext() is used. This way, it can reliably be used to restrict custom menu items to certain documents (Firefox bug 1498896).

• You can now restrict where context menus can appear in an add-on using the newviewTypes property inmenus.create() andmenus.update() (Firefox bug 1416839).

• menus.update() can now be used to update the icon of an existing menu item (Firefox bug 1414566).

• Extensions can now detect which mouse button was used when a menu item was clicked — this can be found using the newbutton property ofmenus.OnClickData (Firefox bug 1469148).

Windows

• Thewindows.create() method now has a new option available —cookieStoreId — which specifies theCookieStoreId to use for all tabs that were created when the window is opened (Firefox bug 1393570).

Privacy

• Theprivacy.websitescookieConfig property is an object that can accept abehavior property — this property can now take a new value,reject_trackers, which instructs the extension to reject tracking cookies (Firefox bug 1493057).

devtools.panels API

• Thedevtools.panels.elementsSidebar.setPage() method is now supported (Firefox bug 1398734).

Manifest changes

• The newpinned property of thepage_action manifest key enables extensions to control whether their page actions should be pinned to the location bar on install or not (Firefox bug 1494135).
• In native manifests on Windows, the 32-bit registry view (Wow6432Node) will be checked first for registry keys, followed by the "native" registry view; you should use whichever is appropriate for your application (Firefox bug 1494709).
• Thechrome_settings_overrides field'ssearch_provider object can now include new properties —suggest_url andsuggest_url_post_params (Firefox bug 1486819), andsearch_url_post_params.

See also

• Hacks release post:Firefox 64 Released