From an interface perspective, this chunk of Fennec is still a work in progress (see some design wireframes here); what you see in alpha 1 is more or less straight out of desktop Firefox. Still, it gets the job done, which means that we can walk through the process:
Above is the AMO page for URL Fixer. Soon AMO will recognize Fennec and offer direct installation, but for the moment, I just had to feed Fennec the XPI. When I did so I got the following:
So, clearly here's a dialog that needs some mobile-design attention. Still, when I tapped "Install Now," it actually went and did so. When I then moved over to the add-ons manager, URL Fixer had already finished installing. (Yes, we need to get some of our browser-level notifications in (as demonstrated over that link in a downloads context), so that users will know that things have happened.) So:
Tapping "Restart Fennec" did what it should. When it came back up, I went and looked in the add-ons manager again, and there it was, safely installed:
And it works!