You can add details of your contacts (including their email addresses) manually: open the Address Book and click the 'new contact' button, or press ctrl+n. If you want to create specific address books (e.g. one for family, one for business) go to the File Menu and select 'new'. You can also set Thunderbird to record automatically all the addresses you write to (including replies and forwards). Go to Preferences > Composition > Addressing and select 'automatically add outgoing email addresses ... ' To send a 'group email' you need to create what Thunderbird calls a ''mailing list''. This is just a list of email addresses (the 'address book' can include email addresses of course but can also store names, postal addresses, phone numbers and other information). To create a mailing list you can either click on the 'new list' button or go to the File Menu in the address book and select 'new'. There's more information here: https://support.mozilla.org/en-US/kb/how-create-and-use-mailing-lists-thunderbird