FF60.6 ESR x64 - I always used this extension to watch YT videos in Firefox (browser-independent, better volume control). It worked perfectly, 5 stars.
For the new Firefox now it needs some extra steps, but it's nothing too difficult.
The problem, however, is that here CMD.exe for security resons is only allowed to run with admin permissions, so this addon it's KO for me unless it will manage to use other methods :\