I don't fully understand this, but I think this is a lot more complicated than it might seem. Because well, you'd first of all need to set up a separate Gecko instance to render the UI, which isn't fundamentally different from what they're doing with tabs, but still an architectural change.

Well, and then you'd probably run into security restrictions, as under normal circumstances, you don't ever want something from inside Gecko to communicate with extensions and probably other things, but for this you need that capability.

Having said that, Mozilla is actually exploring into this direction: https://github.com/browserhtml/browserhtml/blob/master/READM...

And presumably, this is the future, as it would also allow them to get rid of the XUL UI Toolkit, meaning they can entirely focus on core browser technologies. (Which is something different from the XUL/XPCOM extension API that they're deprecating on Tuesday.)