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.