Last I checked, the PST file format wasn't documented sufficiently to read directly, so you'd have to do OLE into Outlook (lots of examples on how to do this). Last time I looked, Thunderbird had no OLE, but you could use the FileRead(), etc... file access functionality in PowerBuilder to read the file using the file format specification (caveat: I've not done this for Thunderbird, so I have no idea how easy or hard it is).
Good luck,
Terry