Oh thank god the developer got this to work with the WebExtensions version of Firefox. I've been dreading the thought of losing this extension for about a year or so now, as I rely heavily on it for productivity. I don't really know what I would do without it - I guess I could have adjusted to Vivaldi's vastly inferior side tab system instead. But now there is no need!

Also, to get rid of top tabs as well, use this workaround: https://www.reddit.com/r/firefox/comments/721ieq/ff57_removehide_tab_bar/

And then go into Customize settings, and at the bottom check 'Tab Bar' to get back the top corner options.