If you're having the same problem, I found an answer on the YouTube help forum. No, do not take crewlesskid's advice and tinker with your Registry. Just listen to YouTube George:
You should only edit your registry if you know what you're doing. You can do some real damage if you make a mistake. Editing the registry may not resolve this issue either.
This specific problem has been a known issue with the Adobe Flash Player. Adobe has released a beta version of Flash 10 that has fixed this issue.
You can read up on it and install Flash 10 beta here:
But before you install Flash 10, make sure you uninstall previous versions of Flash! There is an uninstaller to do this... they don't make the link that visible, so here it is:
That should do it!
And indeed, it did it for me. Be sure to shut down your browser first: the uninstaller didn't mind, but the installer wouldn't proceed until I shut down all of my Firefox windows.
Whew! Now I can back to watching YouTu— I mean, my homework. ;-)