I used to use this extension sometimes and it seemed pretty good. After the "quantum" change in Firefox it did not work. It popped up a message about a "helper app" and i remember it did have something like that which was also required - I think maybe just telling it what converter - like ffmpeg - to use.
Now it points you outside Firefox and Mozilla to a 3rd - 4th? - party site to download an 'installer.' Then you are supposed to install it in your machine.
If you try that through the UBUNTU Software installer you get a lot of warnings about some really bad stuff. This reminds me of what Sony did with the rootkit they foist on unsuspecting users as "a little helper program."
I don't know what that app might or might not do, but it seems risky.