It's possible that you are having a problem with some Firefox add-on that is hindering your Firefox's normal behavior. Have you tried disabling all add-ons (just to check and except Flash, of course), to see if Firefox goes back to normal?

Whenever you have a problem with Firefox, whatever it is, you should make sure it's not caused by one (or more than one) of your installed add-ons, be it an extension, a theme or a plugin. To do that easily and cleanly, run Firefox in safe mode (don't forget to select Disable all add-ons when you start safe mode). Of course, in your case, safe mode doesn't help since it will disable Flash too. In any case, if the problem disappears once you disable every other add-on, you know it's from an add-on. Disable them all in normal mode, and enable them one at a time until you find the source of the problem. See this article for information about troubleshooting extensions and themes and this one for plugins.

If you need support for one of your add-ons, you'll have to contact its author.

If the problem does not disappear when all add-ons are disabled, it's probably something wrong with Flash. Still, tell me, so we can work from there. Please have no fear of following my instructions to the line, as all can be easily undone.

I had this problem also with ALL flash content (e.g. http://www.adobe.com/software/flash/about/) after upgrade to Mozilla 3.6.8.

As the previous post suggested, I disabled all Add-Ons (except Flash player) and it worked OK. I re-enabled all the Add-Ons again one-by-one and it is still working fine.

Thank you both Morbus and ilythgoe, for the help. I tried both your suggestions but to no avail .

I also tried completely uninstalling and removing everything associated with Flash and Firefox (bookmarks,add-ons and customizations) And then reinstalling both.

Results: No hulu and some other flash video sites, in other words, same issues.

Finally I again removed everything associated with Firefox and Flash and then did a clean install of Firefox 4.0 and then Flash.

Results: Works perfectly! Issue resolved!

It seems some Flash video sites are looking for something that was being blocked in Firefox 3.6.8 on my Win7 machine

I had this same problem too. I also have the Flashblock addon installed.

But when I clicked the icon to allow the flash video, nothing would happen.

I finally found that if you had MORE THAN ONE Flash item blocked on the page, you had to click to allow ALL OF THEM before the Hulu episode would play.