The keyboard shortcut for deleting a message is # (Shift + 3). To delete multiple messages, select the messages and use the keyboard shortcut. via DI

If you want to tag the message as spam then the keyboard shortcut is ! (Shift + 1)

Now while trying this out for the first time in Firefox, I hit Ctrl + 3 by mistake, and it turned out to be a Firefox shortcut to select the third tab. So in Windows you can select a particular Firefox tab [1 to 9], by using Ctrl + [1 - 9]