Greasy Fork

YouTube Center Seems Broken : Here the last version (9 Jul 2014) which can be installed.

The GM actual version (v. 2.1.7) don't want to be installed.
It's the same for YouTube CenterVersion: 2.1.7 [OpenUserJS]
And its addon seems broken too:
YouTube Center 2.1.0.1 [Firefox Addon]

It seems a very good userscript, read:
- YouTubeCenter Features
- The last review in its Firefox Addon page (Extremely usefull: by LucasZ 23 March 2017 ):
"This add-on improved functionality of my youtube by 1000%.
VERSION IN MOZILLA STORAGE IS OUTDATED
but developer version from it's website works just fine.
All you need to make it work is Greasemonkey add-on (you don't even have to configure it, just install) before you install YouTube Center from here:
https://github.com/YePpHa/YouTubeCenter/wiki/Developer-Version .
It gives you so many options to customize your YouTube experience that you will never want to browse it without this add-on.
Player resizing, page display customization, no chopped buffering! and many more.
This is awesome"

So, For testing:
▶ Install this Last working release - v.2.1.1 (9 Jul 2014):
YouTubeCenter.user.js [ Install - v.2.1.1 - 9 Jul 2014]
Found in its GitHub page :
YouTubeCenter - Last working release - 9 Jul 2014
OR
▶ YouTubeCenterDeveloper v.548 - YouTubeCenter.user.js [Install]
Found here:
YouTubeCenterDeveloper Version - last: 8 Feb 2017 [GitHub]

I have not tested it now, but its settings panel is visible:
I need to see how it's useful.