Click on the top person in the list of a named address book; hold shift and scroll to the last person and click. Click Write. All the emails in the named address book appear in To:. Finish and send the group email.

ctrl+a is a even better way to select all. Take care to use Bcc to make sure you don't show all your contacts' addresses to one another.

Sending to a whole load of separate addresses is fine if you have less than 60 of them. Thunderbird lets you post to up to "approximately 60" addresses. Beyond this, you'll need to set up a Mailing List. And you may still encounter a limit set by your email provider.