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 Ctr+Shift)

Control Panel > Regional and Language Options > Keyboards and Languages > Change keyboards > Advanced key settings

That seems to have fixed it, thank you so much! I changed it so there are no key combinations assigned, so it won't happen again.

You're welcome