With an upcoming version of WWW::Mechanize::Firefox (likely, 0.620.64, out relatively soon, maybe within this week), it should work like this:

#!perl -w
use strict;
use WWW::Mechanize::Firefox;

my $mech = WWW::Mechanize::Firefox->new();
$mech->get_local("t/52-frameset.html");
print for map { $mech->content( document => $_) }
    $mech->expand_frames('frame[name="myframe1"]');
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But if you only have v0.61, you have to do a bit of work, and change in sub content the line:

    my $d = $self->document; # keep a reference to it!
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to

    my $d = delete $options{ document } || $self->document; # keep a r
+eference to it!
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... and squelch some more warnings. Or duplicate the subroutine ->content and change it to handle arbitrary documents.