Clear the cache and the cookies from sites that cause problems.

"Clear the Cache":

• Tools > Options > Advanced > Network > Offline Storage (Cache): "Clear Now"

"Remove Cookies" from sites causing problems:

• Tools > Options > Privacy > Cookies: "Show Cookies"

Try to disable protected mode in the Flash player.

See:

• [/questions/929081]
• [/questions/928820#answer-340046]
• http://forums.adobe.com/thread/1018071?tstart=0

You can also try to reset Firefox.

• https://support.mozilla.org/kb/reset-firefox-easily-fix-most-problems

If cor-el's solution doesn't work then try this: <It might not be a FlashPlayer issue. It's a possible RealPlayer Plugin issue> Follow these steps if you get this message on Youtube "an error has occured-please try again later": 1) Disable all RealPlayer Plugins or Extensions on FireFox then close Firefox. 2) Launch RealPlayer and click on the RealPlayer icon at the top left. 3) Click Preferences 4) In the Category panel click Download and Recording. 5) Uncheck (remove check mark) from Web Download and Recording and click OK. NOTE: By disabling this you will not be able to download the Youtube video to your computer since the download uses the Realplayer Web download. You can use free software (FREE YOUTUBE DOWNLOAD) by DVDVideoSoft Free Studio at "www.dvdvideosoft.com" to download a Youtube video to your computer if desired. The video will be downloaded into your Video Libraries folder. 6) Start Firefox and try to play the Youtube videos.

This worked for me. I hope it works for you !

guys,I met the same issue,I fixed it and the problem not cause by your PC or youtube,I think you should check a video I found it on youtube,you can get the tips in the video descripion,Its very useful for me

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