I use uBlock Origin, uMatrix and Greasemonkey, and FireFox is nice and fast with several dozen tabs open for weeks at a time.

Slack leaks memory like there's no tomorrow, but that's on Slack. I just close the Slack tab and force a full GC/CC cycle and I get my memory back.

I'd do a binary search of sorts, disable half of the extensions and see if performance is still bad or not etc, to narrow it down to the offending extension(s).