There seems to be such a frustrating variety of versions of this problem
on the web, that I'll avoid being too specific.
I've experienced a number of hangs (and even spontaneous reboots)
on my Windows XPH PC, and it seems to happen on video clips
(youtube and some others), especially if I pause certain videos
and then try to restart.
Among other things, I've updated my video drivers, I've tried
doing without hardware acceleration, and I've very aggressively
uninstalled (and cleaned afterwards) my flash programs and
then installed the latest. None of those worked. But, then I noticed
a) Even in Firefox safemode without any of the addons, I have the problem.
b) But I do not have it in Google Chrome.
c) The trick of appending &fmt=18 to the URL of the video seems to help.
d) similar problems abound and are reported, including at the Mozilla forums,
with varied solutions, a few of them hardware ones. And, some people found
that the problem did not occur in some earlier versions of Firefox.
I'm only intending this post to alert folks, and offer some options.
It looks to me as though a combination of FF and Flash is involved for many,
including for my case. I'm clearly out of my depth.
Even the &fmt=18 trick is puzzling, because all I know about it is that it signals
a higher resolution setting (under what circumstances?),
and that doesn't explain directly why it should be helpful.