So hyped about this. It's the main reason I kept on using Firefox and refused to go back to Chrome or any other browser. The experience is too good, to the point I deleted the YT app and just use YT on FF.
YT on Firefox Mobile has the added benefit of letting you swap to desktop mode and play videos with the screen off, no YT Premium needed ;)
Been doing this for years, but if too much of a hack for you, NewPipe also supports background video, and downloading video, or just the audio, which is great if you have a lot of long form audio-focused channels like music reviews, videoessay content, etc.