Did they ever add actual vertical tab support? When they migrated to webextensions TreeStyleTabs was pretty hamstrung because it couldn't replace the actual tab UI anymore.

I've liked this. Started using this after I got hooked on vertical tabs in Edge at work. https://addons.mozilla.org/en-US/firefox/addon/sidebery/

Yep! This and a few userchrome.css modifications is all it took for me. A few css transitions and hidden elements and viola, it functions better than edge does.

happen to have those css mods handy?