It is possible that you have switched the keyboard layout by accident by using the key sequence to rotate the keyboard layout.

• http://support.microsoft.com/kb/258824 - How to change your keyboard layout

Windows remembers the keyboard layout setting per application and you may have changed the keyboard layout by accident via a keyboard shortcut.

• http://windows.microsoft.com/en-US/windows7/The-Language-bar-overview The Language bar (overview)

• 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 key combination to rotate the layout is the Ctrl+Shift or Alt+Shift combination that is used in Firefox for menu items, so it is possible to change the layout accidentally.
• 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 > Keyboards and Languages > Change keyboards > Advanced key settings > Change key sequence

Thanks a lot, it's worked. I need to read over the lower part of your answer again to understand how to avoid this problem from recurring, but at least now I can work properly in firefox.