Flawless Rated 5 out of 5 stars
This, in my experience, has been flawless.
It currently (v. 1.6.2) works with the latest YouTube/Firefox codes, and the person who develops this add-on is pretty good in keeping up with the updates.
I've never had a problem with this one (I can't say the same for other add-ons).
Bottom line: the only downloader I've found to consistently and reliably work.