After using another tool for YouTube for years, which is no longer being developed, I looked at various alternatives. This one was the best alternative that I liked the most. The only thing that really bothers me is the confusing layout of the settings. Everything is just all over the place.
Update 04/26:
After update from 03. April something did not work anymore with fullscreen view and related videos sidebar. After fullscreen video (back to normal view) the related videos are all under the video and not back on the sidebar.

Clicking a link should NOT turn that link RED. (#FF0000FF)

Many settings do not work now (worked week ago) - example - dimming watched videos on sidebar, full video titles on sidebar

not working. looks like options.html bugged. waterfox g3