Does this happen even if you open a new Youtube tab and then start playing back one single (longer) video, or only in conjunction with autoplay and/or playlists? There's a Firefox bug involving the latter (https://bugzilla.mozilla.org/show_bug.cgi?id=1878115).

Also another thing: If you temporarily try disabling any kinds of battery optimisations and stuff like that (compare https://dontkillmyapp.com/) for Firefox – does that improve the situation in any way?

I've tried some eleven or twelve minute long "Category: Music" videos, and as long as I only play back one single video at a time (in order to avoid the above Firefox bug), everything worked fine on my phone.