drag and drop of an existing tab should be the natural behaviour for side view.
I would use it a lot if it gets implemented but as it is now it is too unpractical.
Thanks for your input. Development on Side View has essentially stopped (aside from converting it to use Shield Utils) and it will become a Shield study later. If it gets good feedback, it might go on to be a feature in Firefox, but at this point we aren’t adding new features to it.