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

Thank you for your reply. Regarding the language bar, although I did not apparently have the multiple language option installed on my Win 7 PC I discovered that by going to CONTROL PANEL/REGION AND LANGUAGE/ KEYBOARDS AND LANGUAGES/CHANGE KEYBOARD/LANGUAGE BAR the mutiple language option was active whilst language display in the task bar option was turned off meaning that the keypress switching action was enabled but un-notified on the screen. Similarly I discovered that the keyboard option for US ENGLISH EXTENDED had been installed somehow. This I removed such that I only have the standard UK keyboard option active and the problem appears to now be cured. Thanks for your help by pointing me toward the eventual answer.

You're welcome