"The hard thing is that i face that CPAN does not handle all dependencies, Isn't it supposed to handle all dependencies automatically?"

Your cpan configuration should be set to follow prerequisites:

cpan> o conf prerequisites_policy follow
cpan> o conf commit
[download]

WWW::Mechanize::Firefox installs fine for me with cpan WWW::Mechanize::Firefox. As you have been told many times you need to set up the MozRepl Firefox plugin, and set it up, in order to run the tests and the module.

Read WWW::Mechanize::Firefox::Troubleshooting and WWW::Mechanize::Firefox::Installation and actually work through the steps given.

If you post another long, wordy yet seemingly content-less stream of though post on this or any other subject, I'm unlikely to reply.

Update: fixed broken link.