Shift<Key>Insert: insert-selection(CLIPBOARD) \n\ <Key>Insert: insert-selection(PRIMARY) \n\ Shift<Btn2Up>: insert-selection(CLIPBOARD) \n\ Shift<Btn1Down>: select-start() \n\ Shift<Btn1Motion>: select-extend() \n\ Shift<Btn1Up>: select-end(CLIPBOARD)

This way, if I select something with the mouse while pressing shift it will go to the clipboard. I can shift-select text on the terminal and paste to Firefox using ctrl-V, for example. And I can copy something from Firefox using ctrl-C and paste it on vim using shift-insert.

Normal selection goes to the other clipboard, so I can paste it using middle button or just clicking insert.