use WWW::Mechanize::Firefox;
my $mech = WWW::Mechanize::Firefox->new(
  launch => '/Applications/Firefox.app/Contents/MacOS/firefox',
);

$mech->get('http://google.com');

##</code><code>##

I, MOZREPL : Client connected : 127.0.0.1:57788 : chrome://browser/content/hiddenWindow.xul
Can't call method "addTab" on an undefined value at /Library/Perl/5.12/Firefox/Application/API40.pm line 131.