Firefox always shows a warning doorhanger if an add-on is installed from a website other than addons.mozilla.org.
What you're seeing is a Mozilla web property following the same rules as every other website. Nothing to worry about.
As 6a68 mentioned, Firefox presents this doorhanger whenever any site other than addons.mozilla.org attempts to install an add-on.
Why is the add-on not hosted on addons.m.o? I thought mozilla thought there was some benefit to users when they only installed things from addons.m.o.
That said, we are also contemplating AMO hosting as an interim step. At the same time, we're having a conversation about how alarming those warning messages look. We were also trying this way to kind of dogfood self-hosted addons outside of AMO. So, lots of things converging on this one thing :/