I have the same problem as well, for me some of the video load fine, but some are very dragging, when i use greasemonkey to download the videos on Youtube, it has normal speed as usual while the video can't load at all. I'm freaking out...

I too am having a difficult time with the videos streaming on youtube. I reverted back to 3.6 and youtube works flawlessly. However, I cannot get my favs toolbar to show any of my favorites. Needless to say, Mozilla needs to come up with a new update pronto.  :-(

BTW, I am on Mac OS X 10.6, and yes, youtube works well with safari.

How did you switch back to 3.6?

I just uninstalled firefox 4, and installed 3.6 again. However, I couldn't get my favs toolbar to work properly. Sry for the late reply.  :-)

Clear all Youtube.com cookies.

my problem is that if i'm full screen on hulu, then go out of full screen, when I try to go back to full screen it freezes & the audio keeps going, but at full screen the video is just black, and on normal viewing the image is frozen. p.i.t.a.

This slow loading problem is almost certainly caused by your virus scanner, most scanners do put more priority on firefox browser than any other ones and the real time scanner delays the loading of the videos 95% of the time.

PLEASE NOTE : I AM NOT ENCOURAGING YOU TO TURN OFF YOUR VIRUS SCANNER!!!!!

but what I normally do is turn off the the real time scanner or the AntiVirus completely, restart firefox and the videos work blazingly fast.

The tip about clearing the YouTube cookies was right on the money! This works in FF6 as well - good call - thanks!