gecko-dev/remote/doc/Building.md

Building

The remote agent is by default not included in Firefox builds. To build it, put this in your mozconfig:

ac_add_options --enable-cdp

This exposes a --remote-debugger flag you can use to start the remote agent:

% ./mach run --setpref "browser.fission.simulate=true" -- --remote-debugger

When you make changes to the XPCOM component 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 RemoteAgent.js, RemoteAgent.manifest, moz.build files, prefs/remote.js, and jar.mn. All the JS modules (files ending with .jsm) are symlinked into the build and can be changed without rebuilding.