@ is on wrong key
While on Firefox the @ symbol shows when I type " and visa versa. It only happens when I am in my own identity on our H/P computer. When I am in my wife's it doesn't do it.
If you have multiple keyboard layout installed in Windows then you may have accidentally switched to another layout that has some keys swapped. Windows remembers the layout per application. The default key sequence to rotate the layout is usually "Alt + Shift" or "Ctrl + Shift" that is easy to trigger with a Firefox command. Best is to disable the shortcut key sequence to switch the layout.