As you asked to transfer the PST files into MBOX files for the Thunderbird, for doing this kind of conversion you can take help of PST to MBOX Conversion tool for Mac OS which has the advanced feature to migrate the data safely and securely. The software I am suggesting is used personally by me and get an accurate result from it. You have a flawless migration with it and no data alteration you will face through it.
Get PST to MBOX Converter to export & convert unlimited PST files of Outlook 2003, 2007, 2010 2013, 2016 to MBOX format with attachments. The software supports all MBOX based email application, it means after successful conversion of PST files to MBOX, any MBOX email client user can view the file without any trouble. Users can download free PST to MBOX Converter software edition to analyze the working process of the software. Visit Here :- http://www.pstfile.org/mbox/