Comment 18 for bug 610915

I would recommend a backout of 447788 entirely. User can also have case sensitive user names that are identical. ie: Username with password A logs in as their administration, username with password B logs in as their standard user account.

With 447788 Firefox is attempting to accommodate a specific set of scenarios while ignoring others specific scenarios. The patch also screws up saved passwords where a site administrator changes from case-insensitive to case-sensitive. Previously entered usernames now become invalid and must be corrected but cannot because Firefox auto-corrects to the saved username/password.

I think a simple logic change may eliminate the issues entirely:

If the password was filled in -> onUserNameFormFieldChange if strToLower(username) === strToLower(stored username) && username !== stored username then prompt "Update|Save As New|Not Now|Never for this site" - if change, update username.

Either way, if a change is made to the username field by the user Firefox should never overwrite this change.