Except that my OS is XP... please help!!!! it used to work so good

A lot of people have been reporting this problem today: Flash works on Adobe's site, but doesn't work on YouTube (or for some people, Facebook). I can't replicate the problem.

Does it play for you in Firefox's Safe Mode? To test...

First, I recommend backing up your Firefox settings in case something goes wrong. See Backing up your information. (You can copy your entire Firefox profile folder somewhere outside of the Mozilla folder.)

Next, close Firefox, hold the Shift key, then restart Firefox. You will get a Safe Mode dialog. Just click "Continue in Safe Mode" and do not "reset" anything just yet.

If YouTube works in Safe Mode, one of your other add-ons (especially in the Extensions category) may be conflicting.

Restarting Firefox in Safe Mode did not make a difference; the problem remains.

Try clearing the Cache in Firefox.

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

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

Youtube may have made some changes recently. Clear the Youtube cookie if above does not do it.

Cleared both cache and cookies; problem remains.

Did you refresh the page after?

If you right-click the black rectangle, do you get the customary Flash player context menu? If not, do you get some other plugin's menu?

One other thought: if you open the plugins page (type or paste about:plugins in the location bar and press Enter), is any plugin registered to handle video/x-flv content? If so, try disabling that one in Tools>Add-ons>Plugins and see whether that helps.

Yes, I refreshed the page. I also tried closing and reopening Firefox; problem persists.

If I right-click the rectangle, I get "about Adobe player." Is that what you mean?

I searched the plug-ins. I did not find any plug-ins for video/x-flv, including Adobe Flash. I don't see that listed. But it must be installed somewhere since I can still see videos on secure sites, no?

Never really experienced issues with Youtube in last day however one video did not want to play even after doing above until I made the url as https://

It's works, I mean the trick to put the "https" instead the classic "http" for youtube.com!

But it's seriously a temporaney workaround, I hope Mozilla fix this youtube problem soon (born with the last 3.6.* firefox update)

I have the same problem - just started. And the https trick works - but cmon Mozilla, let's get this fixed (this is the kind of thing that makes me dread updating firefox).

@nicename, I appreciate your checking the plugins list. I don't have one listed for FLV either, so I think that means that Firefox has no reason to divert the FLV away from the Flash player (which requests the FLV file). Very mysterious.

There are many other threads about this; hopefully when it is solved someone can cross-post the answer here.

Whatever the problem was, it seems to be fixed. I can now play both embedded and videos on the non-secure YouTube site.

It is actually a issue with Youtube and not with Firefox specifically as people with other browsers are getting this same issue also.

http://www.google.com/support/forum/p/youtube/thread?hl=en&tid=3cda4dac122b8f13

I had trouble all weekend with this. It still wasn't working this AM...I tried clearing my cache, and the problem was solved. So, if you haven't already tried it, you might want to give it a shot, it worked for me.