Your above posted list of installed plugins shows that you have the Flash plugin.
You can verify that you have the Flash plugin for Firefox on the Adobe Flash Players Test Page.

http://www.adobe.com/software/flash/about/

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of your add-ons is causing your problem (switch to the DEFAULT theme: Tools > Add-ons > Themes).

See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

thanks a lot for your advice. now,I can play video from youtube site. thanks!!!!!!!!!!

You're welcome

I have the same problem that I can't play any videos in youtube with firefox while I can play them with IE, would you mind helping me as well?

just do as above advices from cor-el

for my problem I just uninstalled firefox theme and youtube will work again!

Hi guys,

I have this same problem too...but I've tried everything under the sun to fix it, and I can't get it fixed.

The dumbest thing about all this is the fact that (to my knowledge) I never changed anything. This problem just suddenly started one day when I was on YouTube.com....as if something on site changed???

Here are some facts...

1.) My Adobe Flash player is completely up-to-date

2.) I can reproduce the problem in safe mode

3.) The problem goes away temporarily when I clear ALL my recent history...and I can usually play about 3 or 4 videos until they quit displaying.

4.) The videos play, but the video screen is just black

5.) This happens ONLY on youtube.com

6.) I ran CCleaner and it did not solve the problem

7.) Whatever the problem is, it goes away when I delete my history (everything)....but comes back as I navigate through YouTube.com and watch videos (usually after 3 or 4 videos)...so it seems as though this might be a cache or cookie problem.

8.) The only thing I have not done yet is to start the browser in safe mode and select the option to "disable plugins" - as Mozilla says this action cannot be undone, I'm a bit hesitant to pull that trigger.

So, I think this has something to do with cookies or cache...but am not sure what, how, or when.

I would really appreciate some help.

I'll gladly supply more info...if needed.

Thanks!

Oh, and 2 things I'd like to add.

1.) I did go through each plugin and disabled each of them individually one at a time, refreshed the video screen to see if anything would change, then went back and "enabled" it again, and tested the next one....and so on down the line until I tested each plugin individually. None seemed to affect the problem.

2.) When I disabled the Shock Wave Flash 10.2.152.32.....the video didn't display at all (of course)....and then when I enabled it again, it acted as if the problem was fixed...the video played just fine...BUT the trick is, it only played fine for 1 round. I tried to play it again the 2nd time, and bingo...I'm back to the same black screen.

Weird, weird, weird.

-)

Do I do the same thing for all the "Extensions" too??? - (they require a FF restart)

I was in the same boat as KenBen. Here's what I did to fix.

Cleared my recent history and then opened a youtube video. It didn't seem to work for the first minute, but then I had to answer a call, and after a minute or two it randomly started working again. So I think all you have to do is open a video, and wait a few minutes. Hope this helps.

I have noticed in addition to not seeing the video part of the YouTube page, like the play/pause and volume buttons, www.Last.FM is all mangled. I am sure I will run into more pages as I go. The only thing I did to change my system was run the Quick Care part of Advanced Systemcare 4.

I have FF5 and Win7. On Youtube,I could hear the audio,but after the black video screen appeared for a second or two,it was gone,with just the white background visible. Finally,I uninstalled the Walnut Theme I recently installed,and bingo! I can now see videos on Youtube again! Sob,and I loved the Walnut Theme!

My problem was with Yahoo! videos. In safe mode they play just fine, but now what do I do? Is there a way to identify what was causing the problem or do I just always load in safe mode?

See https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

[locking thread due to the age]