I have been using Video DownloadHelper for quite a few years and it has become even more indispensable with each new development.
On quite a number of occasions it was able to download files from sites that DownThemAll and iTube Studio could not find or download successfully.

Contrary to some previous posters I have had very little problem with this add-on. As for the watermarking... This is a free-to-use add-on and there are plenty of free convertors out there. I have a paid version of iTube Studio and there are quite a few sites that iTube fails on but Video DownloadHelper downloads with no problem.

If you don't want to pay for the full version, don't, but at least quit bellyaching 'cause a free piece software doesn't meet your exacting standards. There are alternatives - paid and free - go use one of them.

This Add-on is definately one that should be included in any list of Firefox essentials :D