Hi,
I would 'cd' to the top level WWW::Mechanize::Firefox source directory and re-run those failing test scripts individually:

perl -Mblib t/10-plugin-repl-enter.t
perl -Mblib t/16-plugin-repl-home.t
perl -Mblib t/18-plugin-repl-util-help_url_for.t
perl -Mblib t/19-plugin-repl-util-doc_for.t
perl -Mblib t/20-plugin-json.t
[download]

If there's still too much info for the console to neatly accommodate, redirect the output to files, and then peruse those files.

You can install the module anyway, if you wish. You may well find it's quite adequate for your purposes, despite the failing tests. (The choice is yours.)
Should you wish to install it, just run dmake install from that top level source directory.

Cheers,
Rob