There are lots of tools and methods available online to migrate PST files to Thunderbird format but here I want to introduce before you the most effective and reliable PST to MBOX Converter software. It is dedicated tool to migrate PST files to Thunderbird(MBOX) format with complete mailbox data and attachments. The software is capable to migrate highly corrupted, damaged, orphaned or encrypted Outlook PST files to Thunderbird without any kind of data loss. For more info-
https://www.zooksoftware.com/pst-to-mbox/