You may have switched the keyboard layout to a keyboard that has dead keys to allow entering accented characters easily 'a -> á

See:

• http://support.microsoft.com/kb/306993 - HOW TO: Use the Language Bar in Windows XP

It is possible that you have switched the keyboard layout by accident.

• Make sure that you have the Language bar visible on the Windows Taskbar.
• You can do that via the right-click context menu of the Taskbar: Toolbars > Language Bar.

• Check the keyboard language (keyboard layout) setting for the application that has focus via the icon on the Language bar.
• You need to do that while Firefox has focus because Windows remembers the keyboard layout setting per application.
• The default keys to rotate the layout is a combination (Ctrl+Shift or Alt+Shift) that can easily be used in Firefox to activate a menu item.
• To avoid an unintentional switch assign a specific key sequence (Alt/Ctrl+Shift+number) to select keyboard layouts and remove the key combination to rotate layouts (Alt+Shift or Ctrl+Shift)

Control Panel > Regional and Language Options > Languages > Text services and input languages > Details > Key Settings > Change Key Sequence

Note The changing of the key settings for switching the keyboard and the shifting of the keyboard can also be done thru the

   Taskbar:  Toolbars > Language bar 
    Right-Click of Keyboard-Icon > Settings
    Text Services > Settings > Key Settings 
        "To Switch between languages" > (None)

                 and if you want: 
         "To Turn Off Caps Lock" > (*) Press the SHIFT Key

I think