Where does this happen ? Is this about typing in an input field or text area or about something else like the website ? Is it possible that you inadvertently activates your touch pad? ---- It is possible that some status keys have the wrong state and Firefox sees them as still pressed. Try to press all the status keys (Shift, Ctrl, Alt) a few times to see if that helps to recover from this. If it doesn't help then close and restart Firefox or reboot the computer.