I have now marked this issue as solved, even though I have no idea why things went awry
Despite uninstaling and then re installing both IceWeasel and Firefox through the Package Manager, the error I was seeing remained. I used a currency converter and laboriously went through each foreign equivalent to try and determine which currency was equivalent to the NZD amount I was seeing but nothing seemed to "gel".
In the end I uninstaled both Iceweasel and Firefox again and manually deleted the .mozilla configuration file and then installed both from the Software Manager, and copied a backup .mozilla from 27.12.2011. It worked and I am delighted.
So, what caused the error in the first place? I still have no idea.