I have recently upgraded my browser from Firefox 3.6 to Firefox 4 beta version. Now I find that videos are not playing in YOUTUBE website. A black window appears.
Try to install Adobe Flash Again. If this doesn't works, check in you addons manager there isn't any plugin interfering with flash animations/videos
That Firefox 4 beta 9 version is outdated.
Make sure that you update to the latest Firefox 4 RC2 version.
Clear the cache and the cookies from sites that cause problems.
You can also try to change the protocol in the YouTube links from http: to a secure https:
1. START any youtube video that's not working.
2. right-click on face of video.
3. click SETTINGS.
4. UNCHECK "Hardware Acceleration"
I was having this problem and solved it by going into the Add-ons area, clicking on the Plugins menu item and disabling Shockwave Flash 11.3.300.257. Maybe that will help.
Shokweave is disabled by me now videos are playing but speedbit video download is now not working