Thanks! I just installed "keepvideo.7z", and used it successfully. According to the post, that first extension requires Java, but I don't have Java on this machine and the extension works perfectly anyway. In fact, it doesn't even require Javascript.
I went to YouTube with JS blocked, because I didn't want to watch the video on the site. Then I right-clicked a video link and ran the extension under Link Services. It brought up the service page with the proper URL ready for submission.
For my purposes, the toolbar button is unnecessary, since I don't download YouTube videos while they are on the screen. Instead, I use the links in the video search results.
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Gerry