Check that you haven't switched the keyboard layout.
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.

See:

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

• 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 to rotate the layout is a key combination (Ctrl+Shift or Alt+Shift) that you can easily use in Firefox to activate a menu item.
• Disable the key sequence to rotate layouts (Alt+Shift or Ctr+Shift) to avoid an unintentional switch and instead assign a specific key (Alt/Ctrl+Shift+number) to switch to the keyboard layout.

• XP: Control Panel > Regional and Language Options > Languages > Text services and input languages > Details > Key Settings > Change Key Sequence
• Vista: Control Panel > Regional and Language Options > Keyboards and Languages > Change keyboards > Advanced key settings > Change key sequence

Thanks, but the keyboard is fine with all other programs. I deactivated KeyScrambler add-on, and all is now okay with Firefox.