As at this date on Firefox version 57 on Linux Mint 18.3, the updated "companion" is generally needed to download anything .ADP to combine vid and audio two streams. On Youtube it worked very well. A blue button appears whilst it's working. If you reload the download drop down menu and click on the blue button, the particular choice you made will have its progress shown in a window. By clicking on the right hand side arrows for entries, one can delete items to clean up the menu of choices. This is keeps things tidier. The extension keeps being advanced to match the twists and turns of Youtube designed to thwart downloading. Downloading and converting( like mp3 ) is a snap. Always wait a good 6 seconds for any video to be detected. On Ubuntu, Linux Mint and other Debian versions of Linux, the download process for the free and needed companion app is dead easy. If a day comes when the extension doesn't work..give the team a couple of weeks to make adjustments to Youtube's current dodge and you'll find it works again. I find it also works on the latest Firefox in Windows 10. Kudo's once more in its long history.