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/
This SysInspire EDB to PST Converter software provides a secure way to the user for the quick recovery and conversion of the EDB file. This software can work without the need of an Exchange server but keep in mind MS Outlook is already installed in the system of the user before installing the software because it can work on an MS Outlook environment. For the free demo, version users can visit the given link and after satisfying the performance of the software user can buy a premium version of the software.
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Go to the options menu and select Tools. Select Import.
The Import wizard imports your Address Books, Feed Subscriptions, Emails, & Settings. It is possible to import everything or select specific items.
You will need to choose Outlook as the file type to import.
Emails are successfully imported.
This may help, J Wick