I Had the same problem with the button not showing up on the "Firefox 57.0 (64bit) browser"

But then realized that I have my standard firefox (Non-private mode window cookie settings) set only for the old Youtube web page display version) and that this new plug-in has most likely been designed on, & for only Youtubes newest webpage design style, so I went down and decided to test it out by right clicking the "Firefox Icon" below on the taskbar and chose to bring up a new 'Private mode' window (to bypass my cookie settings), and logged-in on the new Youtube layout version (and the button was right there where it was shown to be on the print screen sample) so its possible that some of you who like me don't really like the new version as your normal YT viewing style window, may be having the missing button problem like I was, because of our preference to use the older YT design for our primary viewing times rather then the newer YT version, so you might want to test it out like I did on the 'Private mode window' and bypass your cookies & their settings and see if it pops up for you, because it popped up for me on the newest version, and works fine for me on it once I did that...

Hopefully this will help some of you find the 'missing button' incase you are like I am in cookie settings & the older YT display version normally ;) But if you are missing it on the newest YT display version, I have no idea how to help bring it up for you... :(