Hello TechKnow members and visitors!
Do your YouTube videos give a black screen and play no video?
This can happen after a update of Mozilla FireFox, it demands a newer version of flash. You can check if YouTube doesn't load the plugin by rightclicking the black screen, if the default rightclick menu appears instead of the flashplayer rightclick menu you know enough!
What you need is to update Flash Player, now that can be tricky because the downloader doesnt work all the time, grab the redist version (full offline installer) of Flash Player here: http://www.adobe.com/nl/products/flashplayer/distribution3.html
Update and enjoy YouTube again!