I really hope that people read the recent comments regarding the Youtube code change re: the missing download button. Their quick response to the problem was great (A fix is already uploaded to Firefox for review and approval and should soon get published and the add-on should get auto-updated to v 16.10). This has always worked seamlessly for me.

Work-around for missing download button (thanks Rosa)
...(display YouTube video page)
Tools > Browser Tools > Web Developer Tools (or ctrl-shift-I)
Inspector (tab)> Search HTML (field)> #eytd_list (press enter)
...(displays 1 of 2)
(press enter)
...(displays 2 of 2. box at right: element display: none)
highlight "none" > change to "block"
...(displays popup upper left with download options)
select an option
...(displays "Save As" dialog)

Very useful extension!

GOOD

This app is easy to use and effective.

It worked fine when I first used this addon. But after some days, the download as button is not showing up. So I rated 4 stars.