Sorry I can't help. But I do squabble with YouTube somewhat.
the only special information i have that the video only hangs when it starts to load. it never hung during playing the video.
it would even help me to know if other users have this problem.
I play alot of youtube videos.
I always close firefox completely. Music,video, stops. Firefox browser window closes.
I switch desktops and go back to learning some html.
About 9-17 minutes later, firefox pops up on my taskbar spins for a minute until Im able to click a response from it. the response is the same always; previous session lost / restart firefox / yes or no.
Or firefox is already running,but not responding.to open new window,first close existing process or restart your system.
If i turn off pc before firefox pops up spinning in taskbar, then next time I start Pardus I'll get same message.
This started happening frequently when I started watching alot of youtube videos.
I could probably be right about saying that youtube or adobe flash player plug-ins is more compatible with IE than firefox, thus causing this problem.
NOTE; These experiences I've explained causes no slow downs on my pc. I just notice that sometimes its already running in background somewhere when i want to open firefox.