The Remote Agent is included in the default Firefox build, but only ships on the Firefox Nightly release channel:

% ./mach run --remote-debugging-port

The source code can be found under remote/ in central.

There are two build modes to choose from:

Full build mode

The Remote Agent is included when you build in the usual way:

% ./mach build

When you make changes to XPCOM component files you need to rebuild in order for the changes to take effect. The most efficient way to do this, provided you haven’t touched any compiled code (C++ or Rust):

% ./mach build faster

Component files include the likes of components.conf, RemoteAgent.manifest, files, and All the JS modules (files ending with .sys.mjs) are symlinked into the build and can be changed without rebuilding.

You may also opt out of building all the WebDriver specific components (Marionette, and the Remote Agent) by setting the following flag in your mozconfig:

ac_add_options --disable-webdriver

Artifact mode

You may also use artifact builds when working on the Remote Agent. This fast build mode downloads pre-built components from the Mozilla build servers, rendering local compilation unnecessary. To use them, place this in your mozconfig:

ac_add_options --enable-artifact-builds