This add-on has worked perfectly. There's no watermark and no attempt to download additional programs.
Unlike the leading add-on in this category, it detects videos without having to play them, and it recognizes the titles of the videos. Also, there's no difficulty in selecting which folder to save the download to. Finally, the leading video downloader produced an unviewable, corrupt file when downloading from Vimeo, whereas the file downloaded with "Video Downloader Professional" works perfectly.
Several negative reviews say it doesn't work with Firefox and that there is a charge for it. Both claims are false.
Fails to work with some videos that another extension can download, and successfully downloads some other videos that other extensions cannot download. It seems like I'll have to use two extensions for one purpose.