We were hoping to make that a configurable option for the 3.2 release, but ran out of time. It will be a part of the next release though. If you'd like to hard-code a different format, it's pretty simple though. Simply modify the /plugins/authentication/jfbconnectauth.php file. At line 130, you'll see:
$username = "fb_" . $fbUserId;
To change that to what you ask for above, use:
$username = str_replace(" ", "_", $fullname)."_".$fbUserId;
That replaces the spaces in the user's name with underscores, which is recommended for a username. You can also use just $fbUser (without the str_replace function) if you wanted firstname_1234.
Hope that helps, make sure you test, and good luck!