for me it's just videos on youtube. youtube videos embedded on other pages play.

I was having this problem too. It is simple for Mac - clear the cache. Go to Firefox preferences, network and clear the cache. It should work then...it did for me.

I am getting a black box on Youtube and I am getting nothing at all with videos embedded on Facebook. It just goes white. I kind of regret updating Firefox now.  :/

It was not the Firefox update as some 3.6.* users experiend it on Youtube also.

Try clearing the Cache in Firefox.

Tools-> Clear Recent History -> [v] Cache

or Tools->Options->Advanced->Network -> Offline storage -> [Clear Now]

Clear the Youtube cookie if above does not do it and reload the page. And if the video still by chance still does not load then try making the url in Location (address) bar start with https://

Youtube may have made some changes recently.

James says the truth, I've update firefox with the new 3.6 and youtube didn't work anymore. But the embed works.. And I notice now on youtube appears the avatars in the comments. Some big changes...

Btw, I've try to clear everything and still not working... [sorry for my bad grammar]

It may be a issue with Youtube as people with other browsers are getting the same. http://www.google.com/support/forum/p/youtube/thread?hl=en&tid=3cda4dac122b8f13

I think it's the flash player in general as I get problems on other sites, and it's only when flash is involved. Strangely on youtube i get no problems in 720p and 1080p, if I'm watching something in 480p or less and I change something on the player, like pause etc, it will just hang. The sound will continue but the video will freeze. It will also completely lock up Firefox until the video ends. This happened soon after I updated to FF5. I'm betting Adobe is to blame. Chrome works fine, downloading the videos and watching them in something like VLC is fine. What gets me is that it seems everyone is getting different problems. I'm not getting this black box problem.

This started happening to me just today! IE, Flash won't play with Firefox 5. It starts for a microsecond, then black video area expands and stays black; no video, no nothing. Was fine until today! [ I would like to know before Firefox upgrades itself; especially if changes not tested.] Grrr. I have Windows7; none of the suggestions worked [eg, turning off Adobe 'hardware acceleration', installing an older version of Adobe Flash, blah blah blah. Nothing works!] All is fine in Safari. Aggravating... :-(

I can download videos from youtube, using a firefox addon. But, I cannot view the videos on Youtube.com or on any other website if it is hosted at Youtube.

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.org/kb/Safe+Mode

I updated QuickTime & my Intel graphics card driver (suggested by YouTube), then discovered I was in the middle of an automatic Firefox update to #13. That finally did it for me.

I have firefox 13. This morning when I logged on there was an adobe automatic update. After I installed it I can't watch youtube videos, and embedded youtube videos on youtube won't work either. I note the last two version of Firefox crash very often. Starting to think Firefox is a sinking ship.

I've discovered that watching videos is now a 2-step process. 1st, you click on the > button in the center. Then, click on the > symbol in the lower left position. Without that 2nd click, I get audio, but no video. I'm not thrilled, but I CAN live with it! For whatever reasons, I haven't had near the problems you report with Firefox crashing.

I too now cannot run vids on YouTube and other problems such as loading jpegs into BT Yahoo Mail emails. It's after re-installing Adobe flashplayer as I was having problems.

Is there anyone out there who can tell us how to rectify this? WHERE ARE YOU FIREFOX TEAM?? I don't think they want to talk to us.

Not only is it YouTube, but it's also the case with FarmVille and other games from Facebook. Since I installed the new Flash this afternoon, I have visited FarmVille repeatedly and the progress bar has not moved.

I have tried everything except uninstalling and reinstalling Firefox. That's what I plan to do next. Let's see if that works.

I'll come back with an update.

FarmVille does work on Google Chrome, and I trust YouTube does as well.

There are over a dozen threads about Flash 11.3 and Firefox 13 going in parallel. Hopefully someone will collate all the suggestions into a master list but until then, check out the front page of the forum and see whether any of those are useful. https://support.mozilla.org/en-US/questions

In your Tools > Add-ons (or Firefox > Add-ons) do you have RealPlayer Browser Record or do you use any other similar tools that download videos from the web?

If so, can you disable them and restart Firefox to see if that helps. If not, try disabling Protected Mode in Flash with the following steps: http://forums.adobe.com/thread/1018071?tstart=0

We are working with Adobe right now to find out what exactly is happening.

Update: 6/12/12
Please read the article linked below for more info on this problem:
Flash 11.3 doesn't load video in Firefox

Unfortunately, the uninstall and reinstall did not work. For now, all I can do is play the social games using another browser and trust that the Mozilla team knows about this problem and will work day and night to resolve it.

Thank you. I've disabled the protect mode using the instructions found on your link here: http://forums.adobe.com/thread/1018071?tstart=0

Everything is working fine again.

Steve, I tried it, but got an "access denied" error. I apparently need administrative privileges. How do I get them?