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?

In reply to Re: installation of FireFox :: Mechanize fails - [again] by AppleFritter
in thread installation of FireFox :: Mechanize fails - [again] by Perlbeginner1

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