Personally I never had problems with Firefox breaking pages or having noticeable performance issues. Quite the opposite seems to be the case, since it feels faster and better with every update they released. I really can’t understand all the people constantly complaining about Firefox being slow and buggy. 90% of the time I had issues, it was because of poorly written browser extensions or ad-blockers breaking stuff. It feels like most of the people complaining about Firefox are just looking for excuses to justify why they are too lazy to migrate.
The dangerous thing is when another new Chromium based browser, such as Brave or Edge, appears, which often makes it even worse for the user, since they just share your data with another company.