System Add-ons Overview¶

System add-ons are a method for shipping extensions to Firefox that:

are hidden from the about:addons UI
cannot be user disabled
can be updated restartlessly based on criteria Mozilla sets

Generally these are considered to be built-in features to Firefox, and the fact that they are extensions and can be updated restartlessly are implementation details as far as users are concerned.

If you’d like to ship an add-on with Firefox or as an update (either to an existing feature or as a “hotfix” to patch critical problems in the wild) please contact the GoFaster team: https://mail.mozilla.org/listinfo/gofaster

The add-ons themselves are either legacy Firefox add-ons or WebExtensions. They must be:

restartless
multi-process compatible

Other than these restrictions there is nothing special or different about the extensions themselves.

It is possible to override an installed system add-on by installing a different add-on with the same ID into a higher priority location.

Available locations, starting from the highest priority include:

temporary install (about:debugging)
normal user install into profile (about:addons or AMO/TestPilot/etc.)
system add-on updates
built-in system add-ons

This makes it possible for a developer or user to override a system add-on by installing an add-on with the same ID from AMO or TestPilot or as a temporary add-on.

Default, built-in system add-ons¶

The set of default system add-ons are checked into mozilla-central under ./browser/extensions. These get placed into the features directory of the application directory at build time.

System add-on updates¶

System add-on updates are served via Mozilla’s Automatic Update Service (AUS, aka Balrog). These are installed into the users profile under the features directory.

Updates must be specifically signed by Mozilla - the signature that addons.mozilla.org uses will not work for system add-ons.

As noted above, these updates may override a built-in system add-on, or they may be a new install. Updates are always served as a set - if any add-on in the set fails to install or upgrade, then the whole set fails. This is to leave Firefox in a consistent state.

System add-on updates are removed when the Firefox application version changes, to avoid compatibility problems - for instance a user downgrading to an earlier version of Firefox than the update supports will end up with a disabled update rather than falling back to the built-in version.

Firefox System Add-on Update Protocol¶

This section describes the protocol that Firefox uses when retrieving updates from AUS, and the expected behavior of Firefox based on the updater service’s response.

Update Request¶

To determine what updates to install, Firefox makes an HTTP GET request to AUS once a day via a URL of the form:

https://aus5.mozilla.org/update/3/SystemAddons/%VERSION%/%BUILD_ID%/%BUILD_TARGET%/%LOCALE%/%CHANNEL%/%OS_VERSION%/%DISTRIBUTION%/%DISTRIBUTION_VERSION%/update.xml

The path segments surrounded by % symbols are variable fields that Firefox fills in with information about itself and the environment it’s running in:

VERSION: Firefox version number
BUILD_ID: Build ID
BUILD_TARGET: Build target
LOCALE: Build locale
CHANNEL: Update channel
OS_VERSION: OS Version
DISTRIBUTION: Firefox Distribution
DISTRIBUTION_VERSION: Firefox Distribution version

Update Response¶

AUS should respond with an XML document that looks something like this:

<?xml version="1.0"?>
<updates>
  <addons>
    <addon id="flyweb@mozilla.org" URL="https://ftp.mozilla.org/pub/system-addons/flyweb/flyweb@mozilla.org-1.0.xpi" hashFunction="sha512" hashValue="abcdef123" size="1234" version="1.0"/>
    <addon id="pocket@mozilla.org" URL="https://ftp.mozilla.org/pub/system-addons/pocket/pocket@mozilla.org-1.0.xpi" hashFunction="sha512" hashValue="abcdef123" size="1234" version="1.0"/>
  </addons>
</updates>

The root element is <updates>, used for all updater responses.
The only child of <updates> is <addons>, which represents a list of system add-ons to update.
Within <addons> are several <addon> tags, each one corresponding to a system add-on to update.

<addon> tags must have the following attributes:

id: The extension ID
URL: URL to a signed XPI of the specified add-on version to download
hashFunction: Identifier of the hash function used to generate the hashValue attribute.
hashValue: Hash of the XPI file linked from the URL attribute, calculated using the function specified in the hashValue attribute.
size: Size (in bytes) of the XPI file linked from the URL attribute.
version: Version number of the add-on

Update Behavior¶

After receiving the update response, Firefox modifies the update add-ons according to the following algorithm:

If the <addons> tag is empty (<addons></addons>) in the response, remove all system add-on updates.
If no add-ons were specified in the response (i.e. the <addons> tag is not present), do nothing and finish.
If the update add-on set is equal to the set of add-ons specified in the update response, do nothing and finish.
If the set of default add-ons is equal to the set of add-ons specified in the update response, remove all the update add-ons and finish.
Download each add-on specified in the update response and store them in the “downloaded add-on set”. A failed download must abort the entire system add-on update.
Validate the downloaded add-ons. The following must be true for all downloaded add-ons, or the update process is aborted:
1. The ID and version of the downloaded add-on must match the specified ID or version in the update response.
2. The hash provided in the update response must match the downloaded add-on file.
3. The downloaded add-on file size must match the size given in the update response.
4. The add-on must be compatible with Firefox (i.e. it must not be for a different application, such as Thunderbird).
5. The add-on must be packed (i.e. be an XPI file).
6. The add-on must be restartless.
7. The add-on must be signed by the system add-on root certificate.

Once all downloaded add-ons are validated, install them into the profile directory as part of the update set.

Notes on the update process:

Add-ons are considered “equal” if they have the same ID and version number.

Examples¶

The follow section describes common situations that we have or expect to run into and how the protocol described above handles them.

For simplicity, unless otherwise specified, all examples assume that there are two system add-ons in existence: FlyWeb and Pocket.

Basic¶

A user has Firefox 45, which shipped with FlyWeb 1.0 and Pocket 1.0. We want to update users to FlyWeb 2.0. AUS sends out the following update response:

<updates>
  <addons>
    <addon id="flyweb@mozilla.org" URL="https://ftp.mozilla.org/pub/system-addons/flyweb/flyweb@mozilla.org-2.0.xpi" hashFunction="sha512" hashValue="abcdef123" size="1234" version="2.0"/>
    <addon id="pocket@mozilla.org" URL="https://ftp.mozilla.org/pub/system-addons/pocket/pocket@mozilla.org-1.0.xpi" hashFunction="sha512" hashValue="abcdef123" size="1234" version="1.0"/>
  </addons>
</updates>

Firefox will download FlyWeb 2.0 and Pocket 1.0 and store them in the profile directory.

Missing Add-on¶

A user has Firefox 45, which shipped with FlyWeb 1.0 and Pocket 1.0. We want to update users to FlyWeb 2.0, but accidentally forget to specify Pocket in the update response. AUS sends out the following:

<updates>
  <addons>
    <addon id="flyweb@mozilla.org" URL="https://ftp.mozilla.org/pub/system-addons/flyweb/flyweb@mozilla.org-2.0.xpi" hashFunction="sha512" hashValue="abcdef123" size="1234" version="2.0"/>
  </addons>
</updates>

Firefox will download FlyWeb 2.0 and store it in the profile directory. Pocket 1.0 from the default location will be used.

Remove all system add-on updates¶

A response from AUS with an empty add-on set will remove all system add-on updates:

<updates>
  <addons></addons>
</updates>

Rollout¶

A user has Firefox 45, which shipped with FlyWeb 1.0 and Pocket 1.0. We want to rollout FlyWeb 2.0 at a 10% sample rate. 10% of the time, AUS sends out:

<updates>
  <addons>
    <addon id="flyweb@mozilla.org" URL="https://ftp.mozilla.org/pub/system-addons/flyweb/flyweb@mozilla.org-2.0.xpi" hashFunction="sha512" hashValue="abcdef123" size="1234" version="2.0"/>
    <addon id="pocket@mozilla.org" URL="https://ftp.mozilla.org/pub/system-addons/pocket/pocket@mozilla.org-1.0.xpi" hashFunction="sha512" hashValue="abcdef123" size="1234" version="1.0"/>
  </addons>
</updates>

With this response, Firefox will download Pocket 1.0 and FlyWeb 2.0 and install them into the profile directory.

The other 90% of the time, AUS sends out an empty response:

<updates></updates>

With the empty response, Firefox will not make any changes. This means users who haven’t seen the 10% update response will stay on FlyWeb 1.0, and users who have seen it will stay on FlyWeb 2.0.

Once we’re happy with the rollout and want to switch to 100%, AUS will send the 10% update response to 100% of users, upgrading everyone to FlyWeb 2.0.

Rollback¶

This example continues from the “Rollout” example. If, during the 10% rollout, we find a major issue with FlyWeb 2.0, we want to roll all users back to FlyWeb 1.0. AUS sends out the following:

<updates>
  <addons>
    <addon id="flyweb@mozilla.org" URL="https://ftp.mozilla.org/pub/system-addons/flyweb/flyweb@mozilla.org-1.0.xpi" hashFunction="sha512" hashValue="abcdef123" size="1234" version="1.0"/>
    <addon id="pocket@mozilla.org" URL="https://ftp.mozilla.org/pub/system-addons/pocket/pocket@mozilla.org-1.0.xpi" hashFunction="sha512" hashValue="abcdef123" size="1234" version="1.0"/>
  </addons>
</updates>

For users who have updated, Firefox will download FlyWeb 1.0 and Pocket 1.0 and install them into the profile directory. For users that haven’t yet updated, Firefox will see that the default add-on set matches the set in the update ping and clear the update add-on set.