Try uninstalling Flash and reinstalling it. You are using the non-IE version without active-X, right?

• http://fpdownload.adobe.com/get/flashplayer/current/uninstall_flash_player.exe
• http://www.adobe.com/special/products/flashplayer/fp_distribution3.html

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.com/kb/Safe+Mode

Try to disable the hardware acceleration in the Flash Player.

See:

• https://support.mozilla.com/kb/Cannot+view+full+screen+Flash+videos
• http://www.youtube.com/swf_test.html (right-click the player: Settings)

Flash "Display settings" window:

• http://www.macromedia.com/support/documentation/en/flashplayer/help/help01.html

I thank all of you who replied and cor-el helped me find the solution. For those of you who are experiencing this same problem its probably caused by an extension. For me the problem was AD BLOCK EXTENSION so i removed it and youtube started working properly.

I also found it was Ad Block Plus. I'd blocked an embedded Youtube video from another page which subsequently blocked all Youtube videos.

I disabled Ad blocking rule - ||s.ytimg.com/yt/swfbin/*

Youtube now plays videos again.

Hope this helps.

Same problem - rooted in adblock. I'm on OSX 10.6.8. How I fixed it - When I was on the youtube site I opened Adblock preferences to notify me when a site wanted to access my computer then allowed youtube s.ytimg access which prompted adblock to ask if I wanted to grant access to www.youtube.com. I agreed and presto (after a day of updating plug-ins, replacing every program including firefox and quicktime and silverlight and Flip4mac and flashplayer and reconnecting them all manually, then renewing the pluginreg.dat file in my browser profile folio - all to no avail) I get YouTube player back - with the ADS THAT I NEVER HAD BEFORE!!

I could also have just changed my preferences in Adblock when I was on the YouTube site to allow the site access. You don't have to disable Adblock.

Now is there something changed in Adblock that cuts across basic youtube operation or is there (more likely) something new in YouTube script that links the player intrinsically with the ads so you cannot get one without the other?

Mozilla Adblock developers, good geniuses - get a brainwave PLEASE get rid of the infernal ads. There's a good reason we don't use the other browsers. You are on our side - aren't you?

I don't have Adblock, and all I'm getting from youtube is black screens.