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 > Change key sequence

Cor-el: many thanks, that is exactly what the problem was. Once i opted for the standard US keyboard AND got rid of the Canadian multi-lingual standard keyboard it worked. What threw me was that for some strange reason this problem appeared in Firefox and in Outlook but not in Word ... wierd but now fixed. Many thanks for your help.

You're welcome

Windows remembers the selected keyboard layout for each program, so it is best to have the language bar visible in case you need to switch the layout. Usually having the US International layout that supports deadkeys is sufficient to enter all accented characters, but with the default switching sequence of Alt+Shift and Ctrl+Shift it is too easy to switch the layout by accident.