The new paid desktop app works perfectly -- just like the add-on used to. It's well worth the cost, and the developer is still responsive if you have any problems.

Update: Still the best! Always right on top of the changes to YouTube and quick to give us a solution. Thank you!

Simply the best. The developer is always immediately responsive to changes by YouTube that cause problems, as has happened just recently. Be patient and the best downloader for Firefox will be back!