Try running Firefox in Diagnose Firefox issues using Troubleshoot Mode. If it works in that configuration, then one of your add-ons is the culprit.

I tried running in safe mode and I still did not get any video in Hulu. I am now also having a problem with all flash content not playing in Firefox 4. I have updated all drivers and plugins, reinstalled Shockwave player and Shockwave director player using the Adobe uninstaller. I am running Windows 7 service pack 1 with a Nvidia Ge-force 210 video card with the latest driver. Everything works fine in IE and Chrome. Any suggestions? I have checked the Adobe site for similar problems with no luck. Hulu states they know there is a problem and are working on it. What is wrong with Firefox $?

Could ypu please go to Tools | Add-ons | Plugins and disable Browserplus

Also, go to Tools | Options | Advanced | General and uncheck "Tell websites I do not want to be tracked" if it's enabled.

In the same menu, uncheck "Use hardware acceleration" if it's enabled.

Then close Firefox via File | Exit, wait about 20 seconds and then restart it again.

Thanks for the help, but I found the problem and corrected it. The problem was in the firewall which was changed somehow when updating to Firefox 4 and Adobe Flash Player 10.2.159.1 to allow only outgoing. After changing to allow it works fine. The only browser affected by this was Firefox 4 on my machine. I do not have Browser Plus enabled and using hardware acceleration enabled has not affected its working. Evidently Firefox 4 update makes changes that do not affect other browsers and makes changes to the firewall without asking somehow. Another thing I've noticed is that Firefox 4 does not retain old bookmarks like previous versions have, as I've had to manually input them. Again thanks for the help!

Which firewall are you using as a matter of interest?