The best tool I've used for PST to Thunderbird Conversion is this eSoftTools PST to MBOX Converter Software that is available for free download. It is the best tool I've used in my experience. The steps are really clear, easy, and soft. It shows a live preview of the PST file elements before starting the conversion task. To evaluate the software's usefulness, there is a trial period mode available.
For more information:- Convert PST to MBOX
