If you want to convert PST to MBOX format, then use this eSoftTools PST to MBOX Converter Software. It is a simple and smart method to convert PST to an MBOX file with a single click. The software comes with a free demo version facility by which you can convert PST files into MBOX files for Thunderbird or other MBOX-supported email clients. The free demo has the ability to convert PST files into MBOX with selective 25 items free of cost.
For more information:- Convert PST to MBOX Format