Anecdotally with Firefox on macOS, Gmail is by far the worst offender. Switching inboxes/labels takes several seconds in FF and is near instant in Chrome. Scrolling performance in FF is also much worse.
In my opinion, macOS is definitely FF's weakest desktop platform. I've personally experienced terrible performance, out-of-place UI for the platform (e.g. default dropdowns), and just a generally less polished experience compared to Safari, Chrome, and Chromium derivatives.
I still use it as my default browser on macOS though because it does excel in other areas.
Same for me, I tried the latest Nightly a couple days ago to test the CoreAnimation work, and after 1 hour of use my Slack tab because so unbearably sluggish each keypress would take 1 second to render.
No extensions, no GIFs, no heavy tabs open, perhaps one or two open tabs.
No slowdowns at all after 8 hours on Slack w/ Safari or Brave. Firefox is definitely the worst browser on macOS, compared to the Windows version where it's absolutely perfect and always quick.
