Wow! Easy to use, and does just what it says. Press F1 to show or hide its list. If you wish, it's even possible to hide FF's normal (horizontal) tabs too, though it takes some fussing—see this PCWorld article:
https://www.pcworld.com/article/823939/vertical-tabs-in-firefox-yes-its-really-possible.html
Maybe this extension will eventually add that capability? (If so, they'd want to disable the F1 toggle, because you've got to have tabs somewhere, right?) Thanks for this great idea!