I can almost put my finger on firefox problems. The html/js engine is great. It's the GUI that severly lags. Tabs, menus, url. On the same machine Chromes never stutter, firefox nightly does way too much.
Mozilla did a lot for so long, let's hope they can circle the issues and find resources to fix them.
These are all just anecdotes. I use a low-end laptop from 2010 and Firefox works fine on it. I switched to it from Chrome years ago when they removed some functionality (I think it may have been the backspace to go back but it may have been something else) and I didn't notice any performance issues. If anything it got faster with the Quantum update though I haven't used Chrome since so I don't know how they compare.
I agree with you that it's usable, but on my machine (an i5 iMac/16 gb/Debian) it's slightly slower than Chromium. Not enough that I won't use it, but noticeablely slower.
I also agree with the upstream comments about UI clunkiness. There are a lot of things that irritate me about Firefox that Chromium just handles better.
Mostly little things like the "Recently Closed Tabs" menu is buried 6 levels deep in the "Library" menu. And the completion list that pops up when typing in the address bar always seems to miss pages I've visited but includes things that seem unrelated to what I'm typing in.
Mozilla did a lot for so long, let's hope they can circle the issues and find resources to fix them.