My Windows 7 PC broke so I installed Win10 on a new PC - and found out that I can't activate the classic version in YouTube anymore.
So I copied over my "C:\Users\Name\AppData\Roaming\Mozilla"-Folder from my old PC to the new one and this method restored the classic YouTube View - so I could use this Addon again.

=> If you can*t activate the classic YouTube version find an old "AppData\Roaming\Mozilla"-Folder and replace it with yours. Be advised that this folder also stores all website passwords!