[1] http://www.hackerne.ws/item?id=3739315
[2] https://groups.google.com/forum/?fromgroups#!topic/mozilla.d...
TL;DR: You don't select a BrowserID provider. Your email address determines which provider is used. If your email's domain does not publish the "I am a primary" auto-discovery file, then it falls back to us.
BrowserID provider selection is done with service discovery. When you login to a BrowserID site using an email address, there is auto-discovery logic for checking to see if your email address runs a BrowserID primary.
I believe the fallback to BrowserID.org is a bootstrap thing, rather than the desired long-term goal.
Once Firefox integrates BrowserID support, the user will see a native dialog listing available identities to login to the site, with a checkbox to automatically login in the future.
One of the sites that would pop this dialog is Mozilla Persona. Other sites would too. Your identity is an <email@addre.ss>, and then if you want to sign into Persona and build up a profile around that identity, you can.