Well I finally decided to use ViewTube_GM and VLC as described in my post:
https://trisquel.info/en/forum/viewtube-vlc-abrowserI lastly I wanted to mention that disabling "hardware acceleration" didn't solve the problem, and also, the problem was shown some times even with 360p videos and even without using full screen.
I have seen that this problem has been reported to firefox a lot, so hopefully it would be solved in the near future.
See you all.
