Groups in separate tab bars

[islisis]

I'm not sure how feasible this request is with the current state of Firefox, however as the groundwork is already here in STG I would like to suggest that groups be accessible through separate assignable tab bars. This would all "2D" navigation from a unified interface trackable at a glance.

For shortcut implementation, I imagine the ability to move up/down across tab bars just like it is possible switch tabs in a the standard tab bar. However, an option to allow switching to the last selected tab in each tab bar would allow convenient hopping across tab bars/groups through shortcuts, just like the STG does currently.

The main limitation is that tab bars for a group would not be viewable if that group was currently displayed in a separate Firefox window, according to the current implementation.

I think this would cover the request from #394 .