The thunderbird profile will contain all preferences, mail account settings and Local Folders mail account, downloaded emails, address books, filters, passwords, calendar etc etc. It is everything except the actual program. You will need all that information to get back to that point. Your profile is your mail accounts and emails etc. It is a backup of mail accounts and emails and everything you would naturally assume to be connected to them. You can backup address books separately by exporting address book as LDIF or CSV file. * In Address Book * select an address book * Tools > Export * Choose where to save * Save as type: LDIF or CSV file. * click on 'Save' Thunderbird program is easy to reinstall and is not the same as the profile. Profiles are easy to create and so are mail accounts. But if you have emails in 'Local Folders' or 'Pop' mail accounts then all emails are on your computer in your profile. If you use Imap, then in theory emails are stored on the server. But that is only one copy. I've previously helped people put emails back on a server from a backup, when computer OS crashed whilst Thunderbird was accessing server and server lost emails. So do not assume the one copy of emails on a server does not need backing up.