Are you really sure it’s not because of an add-on? If I remember correctly, Mozilla has said that about 95% of all pages that don’t work aren’t due to Firefox, but to an add-on. I use Firefox exclusively and don’t usually notice that pages don’t work. When that happens, as I said, it’s almost always an add-on that’s to blame. And I dont notice its buggy or laggy. So could be good check your addons next time.
Here are some cases where Firefox really sucks: some of them are specific CSS styles, some are downgraded features, and some of them I just don't know why. As I mentioned here, the ChatGPT web and Gemini web used to be very laggy for no reason—or maybe it was just a bug for me?
I don't think any of this is caused by add-ons, though.
But it's getting better, and most of those problems are just gone;
Still, I keep Chrome around just in case.