Upgrade your Video DownloadHelper — FireFox Extension. You do that by updating your add-ons —in your FireFox add-on menu (in “Tools”).
You need to be up-to-date with version 4.6.2 —it corrects a problem that started with the latest version of YouTube’s website.
P.S.: I have tested extensively all the video capture software packages and services, and Video DownloadHelper is the best. (To convert the FLV files into MP4, use MPEG StreamClip. But you won’t need that most of the time.)
