I was mainly thinking about userChrome.css changes, which allow you to more or less rebuild the whole UI with code. Can't think of many other browsers that let you do that.

Can you do vertical tabs in userChrome.css? Add tab groups/stacks/side-by-side views/workspaces/custom tab context menus?

(not to diminish the css, it's great for theming and correcting the many usability mistakes in browser defaults, and wish all the browsers used that)

Vertical tabs are a native feature of Firefox for a few months now, and before that they were supported with an extension. userChrome.css was (then) used for hiding the original tabs.

I'm not sure what you mean with the context menus, but Web Extensions can add things to context menus.