I recently found a Thunderbird Backup Tool that worked well. It is easy to use, has no restrictions on large file sizes, and offers many more facilities, like removing duplicate emails based on From, To, Cc, Bcc, Subject, and Body. MacSonik Thunderbird Backup Tool is a top-notch utility that can back up and restore Thunderbird mailboxes to the local machine. It helps you to save Thunderbird emails to multiple file formats such as PDF, PST, EML, EMLX, DOCM, DOCX, PNG, JPG, MSG, HTML, MHTML, GIF, etc. Using its mail filter feature, users can download and save selective Thunderbird emails by entering the start/end date.