YouTube changed it's design.
I'm still using Greasemonkey Script "YouTube Center Developer Build 548" and the new YouTube Design just broke it.
Simple "fix" for that:
1. deactivate the script, reload Youtube, so the top bar is shown
2. Click on the 3-button Menu, Restore Classic Youtube
3. re-activate script
Other Firefox Addon recommendations for Youtube:
Enhancer for YouTube™