How to convert PST File to Thunderbird?
I advise you to use eSoftTools Best Free PST to MBOX Converter to convert PST to Thunderbird. PST files can be converted to UNICODE or ANSI. The free demo version is available to all users. In this demo edition, you can convert 25 e-mail items from each PST folder for free.
Read more:- https://www.esofttools.com/pst-to-mbox-converter.html
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Somit Vishwakarma commented
If you don't know how to convert PST file to Thunderbird then simple download MailsDaddy PST converter tool and follow the window to export data in Thunderbird MBOX format. The application supports all Outlook data file (ANSI and Unicode). User can install the migrate data from PST to Thunderbird, Windows live mail etc. The application also convert corrupted and damaged PST file into MBOX format. Get more info: https://www.mailsdaddy.com/blogs/how-to-convert-import-outlook-pst-to-thunderbird/