# 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:

1. temporary install (<about:debugging>)
2. normal user install into profile (<about:addons> or AMO/TestPilot/etc.)
3. system add-on updates
4. 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
<?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:

1. If the `<addons>` tag is empty (`<addons></addons>`) in the response,
   **remove all system add-on updates**.

2. If no add-ons were specified in the response (i.e. the `<addons>` tag
   is not present), do nothing and finish.

3. If the **update** add-on set is equal to the set of add-ons specified in the
   update response, do nothing and finish.

4. 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.

5. 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.

6. 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.

6) 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:

```xml
<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:

```xml
<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*:

```xml
<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:

```xml
<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:

```xml
<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:

```xml
<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.

[balrog]: https://wiki.mozilla.org/Balrog
