I suggest using eSoftTools Best Free PST to MBOX Converter to convert the pst file to Thunderbird. It shows a live preview of pst emails and automatically selects the pst file from your system for conversion. PST files in both ANSI and UNICODE formats can be converted. The free Demo version is available to all users. In the demo edition, you can convert 25 email items to each folder without spending any money, and it can also give services 24 hours a day, seven days a week. Services are also covered.