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.
Know more:- convert PST File to Thunderbird
Convert Outlook to Thunderbird
Find your local . pst and . ost files.
Open your . pst and . ost files.
Convert your emails to mbox or eml format. Select mailbox root folder in All folders pane, and click File > Export. Choose MBOX file format and click Ok.
Find your Thunderbird "Local Folders" Directory. Launch Thunderbird.