Windows remembers the keyboard layout setting per application and you may have changed the keyboard layout by accident via a keyboard shortcut. See: * http://windows.microsoft.com/en-US/windows7/The-Language-bar-overview The Language bar (overview) 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 > Keyboards and Languages > Change keyboards > Advanced key settings > Change key sequence