have been using bazzite for a while now and don’t have issues with Firefox, could be tests Google are running. they recently are targeting users of adblockers and artificially setting high load times in hopes users disable them. a decent alternative is freetube (on flathub) which is anonymous so you can’t login but you can import playlists and includes ad+sponsor block. they’re pretty fast at fixing Google’s attempts to break it too, sometimes you might have to change instances to get it working.

caches are never really a concern to me they will regen after the fact, from your description i would worry more about db, this is dependent though in what you’re using and what you are storing. if the concern is having the same system intact then my primary concern would be backing up any config file you have. in cases of failure you mainly want to protect against data loss, if it takes time to regenerate cache/db that’s time well spent for simplicity of actively maintaining your system