Firefox extensions have their own download process. What is the URL of the extension you are attempting to download?
Have you had Firefox installed since before version 1.5? If so, this link may help you. (requires setting option xpinstall.enabled to true).
Thanks to both of you for your replies. Nanoda, the info you provided did the trick. I did update to 1.5 from an earlier version so that was the problem. Thanks again.