Thunderbird needs a "Get All Messages" button next to the "Get Messages" button at the top of the left-hand pane of servers & folders. It should function the same as the Shift F5 key press or the "All Accounts" options on the "Get New Messages For" submenu option of the File menu on the main dropdown menu.
This would be particularly helpful for people operating Thunderbird with one hand when they have many e-mail accounts. A single button on the surface of the main window would be much easier to use the trying to fish out the operation from the File menu using the mouse or trying to press shift-F5 with one hand.