Workaround: YouTube Center Developer Build (548) broken by new YouTube Material Design

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™
YouTube Plus