This add-on works on other websites but on youtube (which I mainly installed this for) it ALWAYS fails with:
Streams were downloaded but resulted output file is missing or empty. This indicates issues either with downloaded streams (empty or has incorrect format) or with video converter.
The problem might be automatic download since it fails immediately without downloading anything.
I even tried MANUALLY downloading the VIDEO/AUDIO files separately but don't know how to combine them.
Note: I installed the companion application but this add-on doesn't work with youtube unfortunately.
To developer, I pressed ctrl+shift+del in firefox browser 67.0.1 and it brought up the 'Clear All History' window. Here is what I chose to clear - (eg they have checkmarks)
Time Range: Everything
Browsing and Data History
Form & Search History
Offline Website Data
In other words, everything was checked EXCEPT cookies and active logins.
I pressed 'Clear Now' then restarted my browser and went back to youtube. It still fails with the same error message.
Edit 2: I installed this on chrome and it works with chrome. The youtube video is downloaded but it has a .webm extension and I can't open it with anything lol.