Hi Craig,
Thanks for your helpful reply.
I have now done what you have suggested - there has been a little progress but no final resolution to the the problem. This is what has happened:
After running 'Add or Remove Programs', removing the Avast Browser Protection', rebooting the system, installing Avast Browser Protection, rebooting and opening Firefox, Firefox recognized the extension and installed it. It then appeared in the extensions list as enabled but the Avast icon did not appear on the toolbar. Choosing the option to customize the toolbar, there was an Avast Browser Protection icon available but it was grey - nevertheless I dragged it to the toolbar but it remains grey and is not fully functionable. If I click it, a small window opens but it always says 'Unknown Site'.
The problem would seem to be with Firefox and appeared after the update to version 29 and has remained with version 29.1. I use the same version of Firefox in my Windows 7 desktop and there is no problem. I have disabled the only additional Firefox extension I have on my XP system (Saved Password Editor 2.7.2) to see if that is causing a conflict. If that is the case, I will report back.
Thanks for your time.