WebExtensions

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This page is an introduction to Mozilla's implementation of WebExtensions, a new browser extension API. The goals of this API are:

  • It should be easier to use.
  • It must be compatible with multiprocess Firefox (Electrolysis).
  • Porting add-ons to and from other browsers should be easier.
  • Changes to Firefox's internal code should be less likely to break add-ons.
  • It should be easier to review add-ons to reduce the backlog on addons.mozilla.org.

Much of the specifics of the new API are similar to the Blink extension API. Google has extensive documentation on the API. So does Opera.

We hope to ship a preliminary version of this API in Firefox 42.

Packaging

At this point it's difficult to experiment with the API because extensions cannot be installed directly. Bug 1190692 tracks the work to make it possible to install them using the add-on manager. The only way WebExtensions can be installed right now is to package them into XPIs via a Python script. This script generates install.rdf and bootstrap.js files. It can then be zipped into an .xpi and installed using the add-on manager.

In the future, we're planning to use the .zip file extension. All of the data currently in install.rdf will be moved to the manifest.json file. Will will not use install.rdf or bootstrap.js. Extensions will be signed using JAR signing (the same as we do for existing Firefox add-ons).

See https://developer.chrome.com/extensions/manifest for a complete list of manifest directives. The ones we currently support are below.

  • name
  • version
  • description
  • content_scripts
  • permissions
  • web_accessible_resources
  • background
  • browser_action
  • default_locale

In addition, we plan to add the following section Didn't we decide not to do this?: 

"applications": {
  "gecko": {
    "id": "{the-addon-id}",
    "strict_min_version": "40.0.0",
    "strict_max_version": "50.*"
    "update_url": "https://foo/bar"
  }
}

Both versions will be Gecko rapid release versions. The update URL points to an update manifest. We hope to make all of these properties optional. The id would have to be generated fresh either upon local installation or when uploaded to addons.mozilla.org or Marketplace.

List of supported APIs

Note that we expect to fix virtually all of the exceptions printed below.

  • alarms
    • This API is entirely supported.
  • browserAction
    • We don't support the imageData attribute on setIcon.
    • We don't support enable or disable.
  • extension
    • We support only getBackgroundPage and getURL.
  • i18n
    • We support only getMessage in the JavaScript API.
    • We only support @@extension_id and @@ui_locale predefined messages.
    • We don't localize CSS files.
    • Strings to be localized must consist entirely of __MSG_foo__ in order for a substitution to be made.
  • notifications
    • The only supported notification options are iconUrl, title, and message.
    • The only methods we support are create, clear, and getAll.
    • The only event we support is onClosed. We don't provide byUser data.
  • runtime
    • We support onStartup, getManifest, id, and the message passing interfaces (sendMessage, onMessage, onConnect).
  • storage
    • The only storage area we support is local.
    • We don't support getBytesInUse or clear.
  • tabs
    • Unsupported functions: getCurrent, sendRequest, getSelected, duplicate, highlight, move, detectLanguage, captureVisibleTab, get/setZoom, get/setZoomSettings. We probably will never support detectLanguage or captureVisibleTab.
    • We treat highlighted and active as effectively the same since Firefox cannot select multiple tabs.
  • webNavigation
    • We don't support getFrame or getAllFrames.
    • We don't support onCreatedNavigationTarget or onHistoryStateUpdated.
    • We don't support transition types and qualifiers.
    • onReferenceFragmentUpdated also triggers for pushState.
    • Filtering is unsupported.
  • webRequest
    • We don't support handlerBehaviorChanged.
    • We don't support onAuthRequired, onBeforeRedirect, or onErrorOccurred.
    • Requests can be canceled only in onBeforeRequest.
    • Requests can be modified/redirected only in onBeforeSendHeaders.
    • Responses can be modified only in onHeadersReceived.
  • windows
    • onFocusChanged will trigger multiple times for a given focus change.
    • create does not support the focused, type, or state options.
    • update only supports the focused option.

List of APIs we will likely support in the future

Technical details

We currently enforce Chrome's permission model. However, we don't yet support the activeTab permission. We also don't enforce the CSP protections at all. Support for that is planned.

WebExtensions run in the main Firefox process (except for content scripts, which run in the same process as web content). We are planning to run extensions in a separate process (or possibly the content process) eventually.

Additional APIs

We plan to add our own APIs based on the needs of existing Firefox add-ons.

  • NoScript-type functionality. This would come in the form of extensions to webRequest and possibly contentSettings.
  • Sidebars. Opera already supports sidebar functionality; Chrome may soon. We would like to be able to implement Tree Style Tabs or Vertical Tabs by hiding the tab strip and showing a tab sidebar.
  • Toolbars. Firefox has a lot of existing toolbar add-ons.
  • Better keyboard shortcut support. We'd like to support Vimperator-type functionality.
  • Ability to add tabs to about:addons.
  • Ability to modify the tab strip (Tab Mix Plus).
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