Steps to reproduce on FF 4b6: 1. Set up sync for the first time. Select "I have never synced". 2. At the "Create your sync account" screen, select "Use a custom server" from the "Connect to" dropdown. 3. Type the letter "a" into the "User Name" field. Move the cursor to any other field. BUG: Firefox will display (x) Already in use. At this stage, there is no sync server defined, so Firefox is incapable of knowing that the username "a" is already taken, since I haven't specified a sync server yet. My theory is that, lacking a sync server definition, it is checking the username against the Firefox Sync servers. The username "a" is indeed taken in production, but not in stage or dev. Perhaps a potential data privacy issue with people's usernames being sent to us when they specifically intend otherwise.
QA, is this still a problem?
yes, this still occurs. "a" is no longer valid, But I did use a known live account to test against. Note: this occurs if you have selected but have yet to enter a valid custom server URL. If you enter a valid custom server, username (now Email Address) validation works against the custom server.
This bug only likely affects Sync 1.1, and it was shut down in the fall of 2015 (https://blog.mozilla.org/services/2015/07/31/shutting-down-the-legacy-sync-service/).