Opera pretty much has this with it's new tab stacks. I think Firefox panorama is a bit of a misconceived concept where they've gone more for what's "cool" than practical.

The idea with panorama is that the browser is like your desktop and gives you the tool to setup and arrange it like one. But the browser isnt the desktop and there's seldom need for such persistent configurations of browser windows. Furthermore panorama feels one step too removed where you have to go into the panorama view to change group.

Opera's tab stacks feels much more natural and immediate, simply drop tabs you want to group, for instance, a "to read stack" and you can switch to it and it's browser windows immediately. Also pinned tabs are great for putting common windows to the far left, showing only the favicon and preventing it from closing