Grouping/Ungrouping tabs, to my understanding of what you mean, existed in Firefox and was just removed from the browser and pushed into an add-on. It's called "Group Tabs" and I use it extensively. Or I could have this mistaken for 'bundling similar tabs'. I'm not sure on the semantic difference you mean between the two.
Multiple browser windows seems redundant with multiple tabs or the Group Tabs mentioned above.
I use a Firefox addon called FireGestures. Not for the mouse gestures that it is primarily designed for - but because one of the wheel gestures: "[Popup] List All Tabs"
Basically it allows me to hold down right click and use my scroll wheel to switch tabs from anywhere in my browser window. So I hide the tabs up top and get more screen estate! A win-win scenario!
It looks like this: http://i.imgur.com/Wz5Jx9p.png