I have two versions of TenFourFox
One version, the newer, will play music youTubes but is slow to load
and very temperamental.
but once the song starts it plays correctly.
The other older version I use to watch Youtube movies and videos but
it will not play music videos.
Here's a trick I found with the older version:
Open or try to open any YouTube.
Once the page opens use the Youtube search field to search for a movie title.
When the movie is clicked on from the YouTube search results page,
the movie will open as it should.
But no movies will open directly from other URLs or desktop web
icons, without getting that incompatible message.
Music or movie videos WILL open from URLs or desktop Alias icons when
using the newer version of TFF.
I cannot remember or locate whatever I used to log in to the TFF site.