> FWIW, Tree Style Tab solved tab groups (and more) for me.
Yes, but I don't really want tree-styled tabs, I want tab groups :P The interface is vastly different.
Also:
Firefox 64 introduces an entirely new API, browser.menus.overrideContext, which allows complete customization of the context menu shown within add-on content like sidebars, popups, etc.
This is illustrated right in the post by my favorite pain point, the Tree Stye Tab custom tab menu mixing with the regular tab menu.
Finally!
I wish tab handling had a first-class API in Firefox, so that different tab managers could interact with the default tab menu, etc.