Extensions and the add-on ID

Firefox add-ons contain a unique identifier which is used both inside Firefox itself and on the addons.mozilla.org (AMO) website. For example, it's used by Firefox to check for updates to installed add-ons and to identify which objects (such as data stores) are controlled by this add-on.

With older types of Firefox add-on, the add-on developer must set the add-on ID explicitly. XUL/XPCOM add-ons set the ID in the install manifest, while SDK add-ons set it in the package.json.

However, from Firefox 48 you can develop, debug, publish, and update extensions without needing to set an explicit ID at all.

Basic workflow with no add-on ID

Extensions can be set explicitly set the add-on ID using the applications key in manifest.json. However, this key is usually optional. If you don't set it, then you can usually develop, debug, publish, and update your extension without ever having to deal with an ID. One advantage of this is that Google Chrome does not recognize the applications key and will show a warning if you include it.

Note, though, that some WebExtension APIs use the add-on ID and expect it to be the same from one browser session to the next. If you use these APIs in Firefox, then you must set the ID explicitly using the applications key. See When do you need an Add-on ID?.

Developing and debugging

Starting in Firefox 48, if your manifest.json does not contain an ID then the extension will be assigned a randomly-generated temporary ID when you install it in Firefox through about:debugging. If you then reload the extension using the "Reload" button, the same ID will be used. If you then restart Firefox and load the add-on again, it will get a new ID.

If you turn the extension into an .xpi or .zip and install it through about:addons, it will not work. To have it work in this scenario, you will need to add in the applications key in manifest.json

Publishing

Once you have finished developing the extension, you can package it and submit it to AMO for review and signing. If the packaged extension you upload does not contain an ID, AMO will generate one for you. It's only at this point that the add-on will be assigned a permanent ID, which will be embedded in the signed packaged extension.

Note that once an extension has been given a permanent ID, you can't then update it to use the Add-on SDK or legacy XUL/XPCOM techniques. If you do switch to one of these platforms, you must submit it as a distinct new add-on, with a new ID.

Updating

Even after this point, though, you don't generally have to deal with the ID at all. You can continue to develop the add-on without an ID, and when you want to update, upload the new version by visiting the add-on's AMO page. Because you are uploading the add-on through that page, AMO knows that this is an update to this particular add-on, even though it doesn't contain an ID.

You can do the same thing if you are updating from an older add-on type, such as a XUL/XPCOM add-on, to use WebExtensions APIs. Just visit the old add-on's page on AMO, upload the new extension there, and it will be treated as an update to the old version.

When do you need an add-on ID?

• If you are loading the add-on from it's XPI file, are not loading it temporarily using about:debugging and it is not signed.
• If you use AMO's API for uploading your add-on, rather than uploading it manually on its page, then you need to include the add-on's ID in the request.
• Some WebExtension APIs use the add-on ID and expect it to be the same from one browser session to the next. If you use these APIs, then you must set the ID explicitly using the applications key. This applies to the following APIs:
  • storage.sync
  • identity.getRedirectURL
  • Native messaging