My linux PC is pretty old, and it took a while, but this extension was able to convert and save a 35 minute instructional YouTube video to a small mp4 format. It worked well, but I had to scroll down to below the video to see the progress this extension was making in conversion and saving.
There is one bug - if I click on another YouTube video in the same window, and try to convert and save that video, it doesn't -- instead it downloads, converts and saves the original video I viewed in that window, making another copy of it.
It seems like I have to close FireFox before trying to download another video.