I finally fixed the remain issue with the missing flash plugin on my amd64 base pc running ubuntu 7.04 Feisty.
I don’t want to upgrade right now, and swf-player wasn’t working for me.
I decided to follow the steps outlined here (in Italian, and it’s a wiki, so I’m not responsible for what you may found there!)
Come_installare_il_Flash-Player_per_Firefox_(AMD64)_su_Kubuntu