As a user, I'd like to be able to click-and-drag my Firefox Window by grabbing empty Vertical Tab space.
The new vertical tabs (with groups) is great, however it does leave some unfertilised blank space that could be better utilised.
A few times I've clicked-and-dragged on the blank space in the hope of moving my Firefox window somewhere else, such as a different monitor - but at the moment it's just empty space that does nothing.
Firefox