To convert PST files to Thunderbird, I strongly recommend using eSoftTools PST to MBOX Converter Software. It's simple to grasp and use, and it's appropriate for both technical and non-technical people. It's simple to use and understand. It can open pst files and supports all Outlook editions, including 2019, 2016, 2013, 2010, 2007, 2003, and older versions. At the same time, the email attributes will be preserved during the pst to mbox conversion. A free trial version of the software is available. In the free trial version, you can convert 25 items from each folder.
Visit here:- Open a PST File in Thunderbird
If you are looking for solution to convert PST files to MBOX or Thunderbird directly then you can get helps from BitVare PST Converter that is a efficient way to directly convert PST files to Thunderbird (configured) & MBOX format. The software is capable to run on all Windows & Mac both OS. It keeps all mail properties without any change. You can try its free working test edition that allow exporting 10-25 emails from PST to MBOX & Thunderbird at free of cost. https://www.bitvare.com/blog/how-to-convert-pst-files-to-mbox-format/