Well, after some searches, I finally achieved to get full screen flash videos working again !
In fact, I went to:http://www.adobe.com/software/flash/about/
to check if I was using the latest flash version -- which was the case some weaks ago but wasn't the case anymore...
I was using something like v10,1,49,51 and the latest atm is v10,1,53,64...
I tried to use the .deb flash installer which told me I already had the latest version installed...
So, I used the commands :
sudo updatedb && locate libflash
to check where those lib files could've been and I then removed the file ~/.mozilla/plugins/libflashplayer.so, restarted firefox and I was now using the latest version...
Got the first youtube video available to check and the full screen mode worked !
I re-checked the "enable hardware acceleration", restarted firefox again, and the FS mode still worked.
Then, I removed the /etc/adobe/mms.cfg file, restarted firefox, and the FS mode was working again !
So, now, this thread is useless :P
Just check your flash version if you got this issue, they must've fixed something in latest ones.