Note: Since I haven't received a response here, I decided to post on BugZilla too.

I have two accounts, one I don't use too much anymore. I wanted to swap the emails on my accounts, so my newer account would use the email I use more often and my older account would use the other email. I did this and it seemed to work. Firefox Accounts sent the "Primary email updated" notification for both accounts even. However, when I try to login to Firefox Sync/Firefox Accounts now with the new primary email for either account I get a message that says "Primary account email required for sign-in", even though it is the primary email for that account.