You may have turned off an option in Flash preferences to not allow sites to store Flash cookies on your machine. Some videos made in Flash and served from third-party (non-YouTube) pages expect to be able to store Flash cookies, so the videos may not work with this Flash option turned off.

Go to Flash Global Storage Settings Panel which is served from Adobe/Macromedia's site and check the "Allow third-party Flash content..." box, and the box for "Store common Flash components..." where you can then manage sites you have already visited. You'll have to reload the video page after you change this setting.

Also, leave site feedback on those sites where you experienced the problems encouraging them to use HTML5 for serving video content instead of the proprietary Flash format.

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

• https://support.mozilla.com/kb/Troubleshooting+extensions+and+themes

Thanks for the reply sctl! I went to the panel but the boxes were already checked. So alas, the problem is still there.

Thanks for the reply Cor-El! I started in safe-mode and the problem was gone. So after that I disabled my add-ons one at a time and I found out it was my beloved adblocker. Guess what: it was blocking all the youtube-videos...