It is possible that you accidentally switched the keyboard layout.
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.
Windows remembers the keyboard layout setting per application.
The default to switch 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

Solved!

I had the same problem, which only occurs in Firefox but not in any other program, even another Mozilla browser like SeaMonkey.

In the Windows XP Control Panel mentioned above (Regional and Language Options > Languages > Text services and input languages > Details...) I have:

• two keyboard layouts listed under Installed Services, English UK and English US - in that order. The default is set to English UK
• under Advanced tab, System Configuration, I have Turn off advanced text services selected.

I removed the English US keyboard layout which fixes this problem for me.