Backward Compatibility


When making changes to the DevTools, there are certain backward compatibility requirements that we should keep in mind.

In general, we should strive to maintain feature support for existing servers as we continue to make changes to the code base. At times, this can be difficult to achieve, however.

Specific Guidelines

The important compatibility scenarios are:

  • Nightly desktop client MUST maintain existing compatibility back to release channel servers.

This is mainly to simplify cross-platform use cases, i.e. desktop Nightly with release Fennec.

  • Servers MAY use traits to state a feature is not supported yet.

This helps us support alternate environments, which does not implement every possible server feature.

Certainly when a new feature needs a new actor method to function, it won’t work with servers that don’t support it. But we should still ensure the client doesn’t explode when using unrelated, existing features, at least until the above time windows have elapsed.


The harder part of this currently is that there is no automated testing to ensure the above guidelines have been met. While we hope to have this at some point, for now manual testing is needed here.

The easiest way to test this is to check your work against a Firefox for Android device on release channel to ensure existing features in the area you are changing continue to function. That doesn’t cover every case, but it’s a great start.

Alternatively, you can connect to a Firefox release server. This can be done in multiple steps:

  1. Start Firefox release from the command line, specifying the --start-debugger-server with an available port (e.g. /Applications/ --start-debugger-server 6081)

  2. Navigate to a page where you can check that the part of DevTools which is impacted by the patch still works.

  3. Build and run Firefox locally with the patch you want to check

  4. In this build, open an about:debugging tab

  5. On the Network Location section, fill in the host with localhost and the debugger server port you started the Firefox release instance with (e.g. localhost:6081) and hit Enter (or the Add button)

  6. A new item will appear in the sidebar, click on its Connect button.

  7. Accept the Incoming connection popup that appears

  8. Click on the on sidebar item again. You will now see a list of the tabs and workers running in the Firefox release instance. Click on the Inspect button next to them to open a toolbox that is connected to the older server.

Feature Detection

Starting with Firefox 36 (thanks to bug 1069673), you can use actor feature detection to determine which actors exist.

Target hasActor helper

Detecting if the server has an actor: all you need is access to the Toolbox instance, which all panels do, when they get instantiated. Then you can do:

let hasSomeActor ="actorTypeName");

The hasActor method returns a boolean synchronously.


Expose traits on an Actor in order to flag certain features as available or not. For instance if a new method “someMethod” is added to an Actor, expose a “supportsSomeMethod” flag in the traits object for the Actor, set to true. When debugging older servers, the flag will be missing and will default to false.

Traits need to be forwarded to the client, and stored or used by the corresponding Front. There is no unique way of exposing traits, but there are still a few typical patterns found in the codebase.

For Actors using a “form()” method, for which the Front is automatically created by protocol.js, the usual pattern is to add a “traits” property to the form, that contains all the traits for the actor. The Front can then read the traits in its corresponding “form()” method. Example:

For other Actors, there are two options. First option is to define the trait on the Root actor. Those traits will be available both via TargetMixin::getTrait(), and on DevToolsClient.traits. The second option is to implement a “getTraits()” method on the Actor, which will return the traits for the Actor. Example:

Ironically, “getTraits” needs to be handled with backwards compatibility. But there is no way to check that “getTraits” is available on the server other than performing a try catch around the method. See the CompatibilityFront example.

Whenever traits are added, make sure to add a relevant backward compatibility comment so that we know when the trait can be removed.

Maintaining backward compatibility code

When introducing backward compatibility code, a comment should be added for extra information. In order to simplify future code cleanups, the comment should follow the following syntax: // @backward-compat { version XX } Detailed comment, where XX is the Firefox version this code was added in.

Below is a made-up example of what it should look like:

// @backward-compat { version 85 } For older server which don't have the AwesomeActor,
//                                 we have to do this another way.
if (!"awesome")) {

Backward compatibility code can be safely removed when the revision it was added in reaches the release channel. So if something landed in Firefox Nightly 85, it can be removed when Firefox 85 is released, i.e. when Firefox Nightly is 87. Search for the corresponding @backward-compat entries to retrieve all the code that can be removed.