I don't Firefox is moving to Github Actions anytime soon. I was pretty involved with the TaskCluster setup years ago, and it still seems to be running a bunch of CI things.
mozilla-central has a LOT of tests -- each push burns a lot of compute hours.
Yes. All CI is remaining in TaskCluster. Bugs are remaining in bugzilla, not GH issues. Code review is remaining in Phabricator (pre-Phorge fork), pushing in Lando (a custom layer).
Some associated projects are using more GitHub stuff.
mozilla-central has a LOT of tests -- each push burns a lot of compute hours.