I really like Firefox, and used it as my main browser up until a year or two ago. I was a user since way back when it was Firebird. I am that old.
Sometime after the quantum update, it started to become really unstable for me, with multiple crashes per day. I have 16 GB RAM but it seemed to keep running out, every with nothing else open and only 4 or 5 tabs open.
There is an overhead in switching which kept me on Firefox after that probably longer than I should have stayed. I eventually switched to Vivaldi, which I found was more stable and offered many of the Firefox features and customizability which Chrome didn't.
I really want Firefox to succeed, and I think Microsoft missed an opportunity by not basing their new Edge on it instead of Chrome. That would have strengthened a true Chrome alternative so there would be more choice.
Sadly I cannot see Firefox surviving on its dwindling market share, with Google, Microsoft and Apple all backing effectively the same rival code. I know I am using Vivaldi, but I think it's a bad thing for the web if we end up with only one core codebase, even if that is open source.