If you want to convert PST to MBOX Thunderbird, you should try eSoftTools PST to MBOX Converter Software. which is a great option. It has a lot of simple, easy, and fast actions. During the PST to MBOX conversion, email properties are maintained. The software can convert PST to MBOX along with every mailbox item such as - emails, contacts, calendar, attachments, tasks, notes, and more. All consumers have access to the demo version, which is free to use for all users. You can convert up to 25 email items per folder in the demo version.
More information:- https://www.esofttools.com/pst-to-mbox-converter.html