Look into Sideloadly and uYouPlus by qnblackcat . I’ve been using it for years now, and while a hassle to sign the apps every week, it’s the best we have unfortunately.
I find using it in a browser clunky. Much prefer a dedicated app (now I use Tubular), which gives some extra functions like downloads, history and playlists.
As far as I know, Firefox doesn’t have ad blocking or extensions on iOS.
Safari with AdGuard does pretty well though, and there’s Vinegar for YouTube specifically.