maybe a trivial one, but closing the entire browser is via the X in the top right corner, and pressing x on one last tab should not close the entire browser without any prompt. Many times I accidentally closed the entire browser when I wanted to add a new tab and accidentally moved quickly the mouse a bit next to "+". This is especially troublesome when the history is set to clear when closing firefox.
Maybe when closing one last tab, a prompt like: "U're closing the last tab" or: "do you want to close the browser" would be useful?