I've been using Firefox as my primary on both desktop and mobile for about 6 years now, and it's usually pretty great. Desktop rarely has problems. On mobile there are a good number of sites with issues though, because devs don't usually test against it as has had a low number of users. But hopefully this revitalized movement to switch will make them have to care. And that said, 99% of the time these sites are still mostly usable, unless the broken thing is important like say a login screen 😅