Trovata la soluzione e funziona.
change in nsLibero.js the line
var popServerNumber = Math.floor(Math.random()*10) + 1
to
var popServerNumber = Math.floor(Math.random()*4) + 1
START => RUN => %appdata% (Enter) => Thunderbird folder => Profiles
folder => yourprofilename folder => Extensions folder ... Find the
extension forLibero (contains Libero.jar), in the Components folder
you'll find the nsLibero.js file; otherwise, just SEARCH for the
nsLibero.js file on your computer
open the file, search for popServerNumber string, then edit as
suggested by dinolib and save
restart Thunderbird