I would like that tabs and tab groups and MDI and related stuff like that, would be a feature of the window manager rather than the browser. (The application program (e.g. browser) may need to tell the window manager which window the new one is created from, but I would design the operating system to do that.) This way it can be used with many programs and can be used consistently with different programs and does not require add-ons; it also allows to do e.g. split-screen, to use MDI instead of (or in addition to) tabs if the user wants to do, etc.