the resist fingerprinting (introduced via Tor uplift I read) has some downsides for javascript performance (specifically to resist timing fingerprints). If you have it enabled, disable and test if this was causing a slowdown.

Maybe you got the B in A/B testing of a new feature that only saw proper Blink tests. In reddits r/firefox you can get some feedback, also I've seen bugzilla engineers following up on users being able to post performance profiles - a bugzilla search (https://bugzilla.mozilla.org/buglist.cgi?quicksearch=google+... - sort by date desc) could provide you a bugid if you can narrow it down to a specific interaction?