hello Doideira, you can make the video open in the youtube app when you press the little android icon in the right corner of the address bar while you're on the site.

Thank you, Phillip. it would be great if Firefox could tell us to try Youtube App instead of simply (erroneously) saying it can't be played on mobile devices...

Anyway, it's fine... now I know (I hope other people take the time come ask it).

I just realize that it happens to vimeo too, except for the fact that there's no android little icon... and I have no vimeo app installed in my device (and I don't intend to install). And guess what (again)? Android's browser plays vimeo videos with no problem! I don't know if it calls android's video player (since it starts fullscreen right when I touch play button) or whatever, the fact is that Firefox keeps lying to me. it's disappointing... =/