[fixed] Firefox could not download the search plugin from ...

Problem:
Firefox refuses to install a plugin from http://addons.mozilla.org/

Error message:
Firefox could not download the search plugin from:

Root cause:

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.

Fix:
Create an account with addons.mozilla.org and sign in before downloading an experimental addon.