I used the latest release of Firefox 3. for the longest time, long after 4 was out. I upgraded on my windows laptop, and only video stopped working. It is actually like it says here, choppy, but in a special way. I will echo what was said here, the audio is fine.

The video is however not. It plays fast forward, skipping frames, then goes back and replays frames in between, and then goes back and plays more frames in between. When one is watching motion, one sees the same activity forward several times before the playback generally advances to the next scene, and even jumping back and forth between scenes. It seems it is playing all of the frames, just out of order or something. That was my Windows PC. It is now upgraded to Firefox 5.0. I have no resolution to my video problems. CNN videos for instance are terrible.

I made the error of clicking upgrade on my Slackware Linux box, which was running the latest 3. something and upgraded directly to 5.0. Slackware 12 lacked the libraries Firefox needed, but, Firefox installed anyway, and would not open at all. With no way to reverse the upgrade, or downgrade, the computer was toast. I started the box over in Debian Linux. Now Firefox 5.0 works, but it will not play videos at all. I wish I had stayed with the latest release of Firefox 3.

In summary, as a user, I have seen no advantage in upgrading over Firefox 3 -- nothing I need in 4 or 5, but that would be OK, if 4 and/or 5 had not broken video playback.

Fixed it (kind of).

<edit> I just realized you aren't running Linux. Give what I did below a whirl anyway. It may be related. </edit>

I had the same issue with Firefox 4.x, 5.x, 6.x, and at the time of writing 7.x. I was using version 3.6 and then that one stopped working too. I rolled back to 3.5 and then it worked again. Each installation is in its own directory.

It did not matter what version of Flash, and I'm currently using 10.3r181. Nothing wrong with CPU spikes or memory spikes or buffer settings or permissions. I'm running Mandriva 2010 and KDE 4.3.

For plugins, I'm using Flashblock and Adblock.

I tried all kinds of troubleshooting, including removing and disabling different plugins. I removed everything from ~.mozilla thinking there was a problem in there. I sudo'd over to root and ran Firefox from there to force the recreation of the temp files. I removed everything from ~/tmp. None of this worked.

I logged out, created a new user account, and logged in with that account....and voila, it worked. I put Adblock and Flashblock back in, and all is good This tells me it's something to do with the X session and some of its temp files that is misbehaving, but I have given up looking; there are lots of temp folders in ~ and /tmp relating to X, and I wouldn't even know where to start.

I moved my profile over and now I login with my new user, and all is good.