I am very disappointed with this add-on. when I find a YouTube video and I click on this icon in my toolbar, the download button is missing and the message I get is:

Downloading this video is not possible with a Firefox add-on !
The YouTube website contains only an incomplete video address, so the video can be downloaded only by YouTube itself or by a special program.
Video Downloader Ultimate for Microsoft Windows is such a program and you can find it here.

All it is, is an ad to get you to spend money and buy their software.