Sorry Eugenio, I obviously didn't make myself very clear or I'm misunderstanding your need. They are automatically synchronised with your Android phone and available in Calendars, Apps and the Phone. I bet you wouldn't even notice the difference from using Google Contacts.
With regards to backups, already done! We have full versioning so that you can roll back to anytime you want and backups are done continuously.
Yep - your contacts can be easily transferred from Google Contacts and then used on your Android phone and in Thunderbird just the same.