Hi,

I have just checked it again:
1) delete old Firefox;
2) delete old Firefox Profiles from C:\Documents and Settings\\Application Data\Mozilla
3) download and install the latest english version from a link at www.mozilla.com (firefox.exe from 11 oct. 2006);
4) open page [Link: addons.mozilla.org] and install add-on;
5) restart Firefox;
Add-on works.

May be your old profiles make problems. Could you please reinstall Firefox according these steps?

Best Regards,
hugbug.