First login is entirely based off the condition of [lastvisitdate] Equal To 0000-00-00 00:00:00. So you can probably use a reverse condition of that like [lastvisitdate] Not Equal To 0000-00-00 00:00:00, but I can't recall if the last visit date will be updated before or after that trigger.
$firstLogin variable should just be a part of that trigger to make it easier though. I've added a feature ticket to improve the login triggers with that variable in a future release.
forge.joomlapolis.com/issues/6458