Rated 5 out of 5 stars
Like the others, I noticed that it stopped working recently. It happens - FF changes something or YouTube changes something, and it takes some time for the addon to catch up. It's not as if they send out notices ahead of time, especially since YouTube would certainly prefer to make it impossible to download anything.
That said, the below suggestion to try the Developer Channel beta version was an excellent idea. I switched to that version, and the downloads showed back up upon restart. They're a little odd; the MP3 format downloaded the video and everything (which works okay, though it's weird to have an MP3 video), and the Full HD MP4 came out glitchy and truncated (which doesn't work, but the MP3 gave me video...) I tried renaming the extension to see if the MP3 video still works as an MP4, and so far it's fine. It's a bit roundabout, but maybe this detail will help refine the update.
A suggestion for future options - perhaps the clipping could be made to work at either end? There are frustrating videos with long silences at the end instead of the beginning, and it would be nice to be able to easily cut the dead air off.