This bit:

you may need to install the WWW::Mechanize::Firefox module
[download]

gives a valuable clue: fire up a cpan shell, and install WWW::Mechanize::Firefox:

$ cpan

cpan shell -- CPAN exploration and modules installation (v2.05)
Enter 'h' for help.

cpan[1]> install WWW::Mechanize::Firefox
...
[download]

Have you tried that?