No this was just pseudo code to try to set an example. I have pre-created 64 FireFox profiles each with Mozrepl running on a different port. $ff_profile is picked in a DBI call and $ff_port is a sum of that and equals the Mozrepl setting.
I've watched it run. It visually appears as if it is running perfectly. I've limited the number of forks to 25 (5x5) and WWW::Mechanize::Firefox appears to do every method properly. Except that when a set of forks goes to work on cases with the same $url, the $html for each fork is a jumble of the $html of other forks. This result in a temporary html with a mix of case details, which end up in the proper PDF.