I tried FF38.0.1 from the distribution repos and 38.0.6 from the community
repo. With both versions, when I tried to go to youtube.com, the site loaded
(the front page was completely rendered) but the cursor currently active (i.e.
the hand cursor when hovering a link) doesn't change anymore while in the
firefox window. No other tabs can be opened, the window is unusable.
This does not happen with the firefox I downloaded from the mozilla site as a
tar.gz package.
Notice that I don't even try to play a video but I only visit the front page. I
also tried this with a blank account (i.e. mv ~/.mozilla ~/.mozilla.bak and
start ff afterwards).