Restore Old Youtube has been blocked for your protection.

Why was it blocked?
This add-ons does remote script injection in a way that attempts to hide the injection. The code being executed runs on all pages which is contrary to what the add-on is described to do.
What does this mean?

The problematic add-on or plugin will be automatically disabled and no longer usable.

When Mozilla becomes aware of add-ons, plugins, or other third-party software that seriously compromises Firefox security, stability, or performance and meets certain criteria, the software may be blocked from general use. For more information, please read this support article.