It is possible that you accidentally switched the keyboard layout. You can check the keyboard language (keyboard layout) setting for the application that has focus via the icon on the Language bar (Taskbar, right-click context menu: Toolbars). You need to do that while Firefox has focus. Windows remembers that setting per application and the default 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. For XP see: Control Panel > Regional and Language Options > Languages > Text services and input languages > Details > Key Settings > Change Key Sequence For Vista see: Control Panel > Regional and Language Options > Keyboards and Languages > Change keyboards > Advanced key settings > Change key sequence ..... http://support.microsoft.com/kb/258824 - How to change your keyboard layout ..... http://support.microsoft.com/kb/306993 - HOW TO: Use the Language Bar in Windows XP http://windows.microsoft.com/en-US/windows-vista/The-Language-bar-overview - The Language bar (overview)