Firefox refuses to install a plugin from http://addons.mozilla.org/
Firefox could not download the search plugin from:
If you are downloading an experimental addon, Mozilla.org requires you to Log-in. with an account. When you click on "Install addon" Firefox expects to read the XML description document from the link, but mozilla.org points it to the login page instead.
Create an account with addons.mozilla.org and sign in before downloading an experimental addon.