Ah, right, this will break the multi provider case, good catch. I implemented this in a rush a few hours before the demo. The reason why we needed a preference UI for this is that to demo offline, we used an ad-hoc wifi and our IP addresses changed each time we reconnected. Editing the preferences in about:config was too slow and too error prone. Even if we don't care specifically about the ad hoc wifi case, I think being able to change the demo server hostname without going to about:config is useful (especially if we want to test sometimes localhost, sometimes on the demo server and petri, sometimes on Romain's installation on petri, etc...)
My first implementation had a preference observer, but it didn't work for some reason that I couldn't figure out in the limited time I had. I guess I should try it again.