Tab groups like in the reply below, but also a number of other things such as a fully functional revival of DownThemAll (the webextension version still isn’t as powerful), and a number of other useful addons from here.
Plus, I like to write my own complete themes, and it’s far more convenient to package them into installable XPI files than it is to just use userChrome.css, since they can be easily distributed and updated automatically.
Tab groups like in the reply below, but also a number of other things such as a fully functional revival of DownThemAll (the webextension version still isn’t as powerful), and a number of other useful addons from here.
Plus, I like to write my own complete themes, and it’s far more convenient to package them into installable XPI files than it is to just use userChrome.css, since they can be easily distributed and updated automatically.