There are many other third-party tools available in the web market but only a few of them are reliable. I want to suggest you use the MacSonik Mac Thunderbird Backup Tool. This tool helps to save Thunderbird emails to PDF, PST, EML, EMLX, DOCM, DOCX, PNG, JPG, MSG, HTML, MHTML, GIF, etc. It can backup Mozilla Thunderbird emails to 20+ formats and its prominent feature of Eliminate Duplicate Email is outstanding. Using this feature you can remove identical emails from the backup process of Thunderbird emails.