Marionette is a remote protocol that lets out-of-process programs communicate with, instrument, and control Gecko-based browsers.
Within the central tree, Marionette is used in most TaskCluster test jobs to instrument Gecko. It can additionally be used to write different kinds of functional tests:
Marionette supports to various degrees all the Gecko based applications, including Firefox, Thunderbird, Fennec, and Fenix.
Some further documentation can be found here:
Bugs are tracked in the Testing :: Marionette component.
If you prefer real-time chat, there is often someone in the #interop IRC channel on irc.mozilla.org. Don’t ask if you may ask a question; just go ahead and ask, and please wait for an answer as we might not be in your timezone.