Try the eSoftTools PST to MBOX Converter Software if you do not know how to open a PST File in Thunderbird so you may use this tool. It can easily convert a PST File to Thunderbird. It performs very simply and smoothly. With this software, you can easily convert pst to thunderbird. it comes with free trial editions to convert few items from pst to mbox.
Visit here:- Open PST File in Thunderbird