Run the Firefox program once as Administrator (right-click: Run as Administrator).

If that isn't possible then you need to remove the download update.

See http://kb.mozillazine.org/Software_Update (Software Update not working properly)

Remove the files in the updates and updates\0 folder:

C:\Users\<user>\AppData\Local\Mozilla\Firefox\Mozilla Firefox\updates
(%LOCALAPPDATA%\Mozilla\Firefox\Mozilla Firefox\updates)

Thanks for that, finally got the time to try out those solutions!

Running as an Administrator didn't work. Not sure why. I'm not really that knowledgeable about technology!!!

There are 3 seperate users on the machine in question including an "Admin". The problem only related to my own account. I logged on to my own account and then ran Mozilla as the Admin but that only opened the Admin's Mozilla, which of course didn't have any error.

However, removing the suggested files as above did the trick!!

This problem happens when a non-administrator user-id downloads the update via automatic updates. (In windows)

When the update is applied, the system asks for the administrator password. The install completes but the update files under the original non-administrator id are retained, resulting in the erroneous message.

The fix would seem to be straight forward. When FF detects the new version (it does this and displays the "what's new in version XXX" page, the update should be deleted. Or on install, updates for all impacted users (or even the update that was just installed) could be deleted.

Thunderbird exhibits exactly the same - erroneous - behavior.

This happens in ever version/release update. Every. Single. Time. Fix the damn update process.

Or don't allow non-administrator users to select auto-update.

Either way, this is a stupid problem that should have been fixed months ago!

I found I also had to delete the files

active-update.xml

  and

updates.xml

  in 

%LOCALAPPDATA%\Mozilla\Firefox\Mozilla Firefox\

for the affected user.