1. Namespace
2. Asynchronous
3. API Coverage
4. Manifest keys
5. More information

Firefox is the most compliant with the proposed standard, and is, therefore, your best place to start when developing browser extensions. A simple way of addressing many of these differences is by using theweb extension polyfill library. For an introduction to using this tool, seeBuilding a cross-browser extension. Note, however, that the polyfill does not support Firefox exclusive APIs that are not available in Chrome.

In Chrome, asynchronous APIs use callbacks to return values, andruntime.lastError to communicate errors:

functionlogCookie(c){if(chrome.extension.lastError){    console.error(chrome.extension.lastError);}else{    console.log(c);}}chrome.cookies.set({url:"https://developer.mozilla.org/"}, logCookie);

The equivalent WebExtensions API code usingpromise:

functionlogCookie(c){  console.log(c);}functionlogError(e){  console.error(e);}var setCookie= browser.cookies.set({url:"https://developer.mozilla.org/"});setCookie.then(logCookie, logError);

If you are unfamiliar with how JavaScript can handle asynchronous events or promises, take a look atGetting to know asynchronous #"https://developer.mozilla.org/docs/Web/JavaScript/Guide/Using_promises">Using promises page.

• Variations in the support for features within a function. For example, at the time of writing, Firefox doesn’t support thenotification function methodonButtonClicked while Firefox is the only browser that supportsonShown.

• Proprietary functions, supporting browser-specific features. For example, at the time of writing, containers is a Firefox-specific feature supported by thecontextualIdentities function.

Full details of the differences in API support are provided inBrowser support for JavaScript APIs on MDN. Where there are caveats regarding support for an API feature, there is a drop down below the version number that expands to show details of the caveat. The API features reference page also explains the caveat.

The differences in the supportedmanifest.json keys among the browsers fall broadly into two categories:

• Extension information attributes. For example, at the time of writing, Firefox and Opera include thedeveloper key enabling the addition of details about the developer of the extension, as well as the author, to be recorded.
• Extension features. For example, at the time of writing, Edge did not support thecommands key that enables shortcut keys to be defined for an extension.

Full details of the differences in key support are provided in theBrowser compatibility section of themanifest.json page on MDN. Where there are caveats regarding support for a manifest key, there is a drop down below the version number that expands to show details of the caveat. The manifest key reference page also explains the caveat.

Variations due to browser behavior

While a web extension API may be compatible between Firefox and the Chromium-based browsers, variations in the browser behavior may mean that the outcome for an extension or to the user are not identical.

Cases where the extension behavior may be affected include:

• URLs in CSS: Firefox resolves URLs in injected CSS files relative to the CSS file itself, rather than to the page it's injected into.
• Web accessible resources: These assets use a URL with a browser-specific scheme where the authority is an extension-specific identifier, where:
  • Firefox uses the internal UUID, a unique identifier assigned randomly to an extension when it's installed.
  • Safari uses the internal ID, a unique identifier assigned randomly to an extension per browser session.
  • Extensions for Chromium-based browsers use an ID assigned to the extension when it is published. However, thekey property can be used to set this ID.
• Content script requests context: In Chromium-based browsers, when a request is called (for example, usingfetch()) to relative a URL, such as/api, from a content script, it is sent tohttps://example.com/api. Firefox uses absolute URLs.
• Native messaging: there are variations in the command-line arguments, manifest key names, and manifest location between Firefox and the Chromium-based browsers.

For example, in Firefox notifications are cleared immediately when the user clicks them. This is not the case in Chrome.

Details of these variations are documented as part of the API feature reference pages.

• Browser support for JavaScript APIs
• Browser compatibility for manifest.json

Tags: beginner extensions webextensions compatibility cross-browser 

Contributors: rebloor 

Last update: rebloor May 27, 2019