firefox-addon Getting started with firefox-addon Introduction



Firefox add-ons are generally grouped into Extensions, and then "other types" of Firefox add-ons.


Extensions allow Firefox to be customized by adding to or modifying the functionality of Firefox. Some of the types of things which can be done with extensions include:

  • Change how specific websites appear, their content, or how they are interacted with.
  • Customize the Firefox user interface
  • Add additional features to Firefox
  • Change how existing Firefox features function

Firefox extensions are, primarily, written in JavaScript with the addition of some JavaScript APIs.

Deprecation and removal of all types of extensions other than WebExtensions

Firefox add-ons, particularly extensions, are in a state of flux at the moment. Mozilla has announced, and confirmed, that they have deprecated all types of Firefox extensions, except WebExtensions, and that all non-WebExtensions based extensions will be disabled in Firefox 57, which is scheduled for 2017-11-14.

Types of Extensions

Firefox has four types of extensions (all of which are commonly referred to as add-ons):

  • WebExtensions: Moving forward, WebExtensions are the only type of Firefox extension which will be supported. These add-ons are described by a manifest.json file. This API is similar to what is used for Google Chrome extensions. These add-ons use HTML and CSS in addition to Javascript. While Mozilla has stated that this API is the future of Firefox extensions, this API is still in development. For now, you are probably best off developing and testing your WebExtension add-on with Firefox Developer Edition, or Firefox Nightly. You should also make careful note of what version of Firefox is required for the functionality you desire to use. This information is contained in the "Browser compatibility" section of the MDN documentation pages.

    WebExtensions use a significantly different API than the other three types of extensions. There is, intentionally, no ability to use the interfaces provided by any of the other add-on types.

  • Add-on SDK: [deprecated; scheduled for removal] These add-ons are described by a package.json file which is initially generated by executing jpm init. These extensions will often use require() to load either High-Level, or Low-Level APIs to interface with Firefox. These add-ons use HTML and CSS in addition to Javascript. Currently, these add-ons are wrapped into a bootstrapped extension when they are loaded for testing by jpm run or consolidated into an .xpi file by jpm xpi for distribution (i.e. upload to AMO/Mozilla). In other words, they are bootstrapped extensions with an SDK wrapper.

    Mozilla appears to be committed to continuing to support Add-on SDK based extensions as long as the extension does not use require("chrome"), or otherwise depend on XUL, XPCOM, and XBL.

    Most of the things that can be done in a bootstrapped extension can be done in an Add-on SDK based one. However, many such things bypass the SDK which forfeits a significant portion of the benefits of using the Add-on SDK.

  • Bootstrapped: [deprecated; scheduled for removal] These extensions are also commonly called "restartless" because they were the first type of Mozilla extension which did not require the application to be restarted in order to load/unload the add-on. However, restartless is a descriptor of how they function. Using "restartless" as the name for this type of add-on is confusing because both Add-on SDK and WebExtension add-ons also do not require the application to be restarted upon load or unload of the add-on. For that reason, there is a tendency to no longer use "restartless" as the name for this type of add-on.

    These add-ons use HTML and CSS in addition to Javascript. Many also interact with Firefox using XUL.

    These add-ons have a JavaScript file called bootstrap.js which must contain entry points (functions) which are called for add-on startup(), shutdown(), install() and uninstall().

    These add-ons contain a install.rdf file that describes the add-on. They usually, but not always, also contain a chrome.manifest file that describes how the files and directories in the extension relate to the Mozilla application (e.g. Firefox).

    Most, but not all, of the things that can be done in overlay/XUL/Legacy extensions can be accomplished in bootstrapped add-ons. Anything that can be done in the Add-on SDK can be done in a bootstrapped extension (Add-on SDK extensions are bootstrapped add-ons with some JavaScript based API layers).

    Mozilla has stated that they plan to deprecate "add-ons that depend on XUL, XPCOM, and XBL." While not all bootstrapped add-ons depend on these technologies, there is a tendency for for bootstrapped add-ons to operate at a lower level than Add-on SDK and WebExtension add-ons. Thus, they are more likely to use these technologies. While there are some that are saying that all bootstrapped add-ons are planned to be deprecated, it is not clear that is the case. After all, Add-on SDK extensions are not being deprecated (unless they use require("chrome"), or otherwise depend on XUL, XPCOM, or XBL) and all Add-on SDK extensions are bootstrapped extensions, just with an SDK wrapper.

  • Overlay/XUL/Legacy: [deprecated; scheduled for removal] These add-ons contain a install.rdf file that describes the add-on and a chrome.manifest file to describe how the add-on's files relate to (e.g. overlay) the application's files. How the add-on functions with the application is completely dependent on the relationships described in the chrome.manifest file. The only exceptions to this are a few things like icons for the extension and the file describing the extension's options which are indicated in the install.rdf file. These extensions interact with the application (e.g. Firefox) at a very low level. This tends to make them more likely to break when changes are made to the application.

    These add-ons use XUL, HTML and CSS in addition to Javascript. Some also use XPCOM, and XBL.

    All Overlay/XUL/Legacy extensions are planned to be deprecated.

Other Types of Firefox Add-ons

When most people think about Firefox add-ons they are thinking about the extensions described above. However, there are some additional types of Firefox add-ons:

This "example" is primarily copied, with some modifications, from my, Makyen's, answer on a stackoverflow question.
Some portions of this content were copied, or at least based upon, the Add-on page on Mozilla Developer Network (MDN).
This was originally posted by Makyen in the "Remarks" section of the firefox-addon tag. It was then modified by Ondřej Doněk, who removed an extra "for ". In a single edit performed by nus, it was moved from "Remarks" to a pinned "example" titled "Introduction". Unfortunately, doing so resulted in the system loosing attribution information.