Mozilla

If you need to get an insecure extension working in Firefox 3, you can pull up "about:config" and create a new boolean variable named extensions.checkUpdateSecurity set to FALSE. This will allow the extension to run, but disables a good safety feature.

The ideal way to fix this, is to contact the extension author and ask them to place their extension on a secured site.

You can read more about this issue from a post of mine in November, the Mozilla wiki, and the bug entry in BugZilla.