Hi Essentially, you write en email, then add all the recipient email addresses to send the same message to all of them. Read this article https://support.mozilla.org/en-US/kb/addressing-email especially the part on BCC When you are sending the same email to many people, it is not recommended to put all the email addresses in TO: - because that means all the recipients will see all the addresses of all the other people. Sharing people's email addresses without permission can get you into trouble. To make the addresses hidden, write the email, put your own email address in TO:, then add all the other recipients to BCC as mentioned in the article. If you are planning to send emails to the same group of people in future, you may create a 'group' in your address book, so as to select the same group of recipients more easily.

Further information about creating a group for mass emails here https://support.mozilla.org/en-US/kb/how-create-and-use-mailing-lists-thunderbird