If you want a free and reliable solution this Thunderbird to Outlook Converter by CubexSoft to convert your Thunderbird emails directly to PST without needing to be reliable on extra software like Thunderbird or Outlook cause it directly transfers all the emails in PST format while maintain data accuracy and the chain of folder hierarchy. It comes with various popular file formats so you can change to MBOX, PST, MSG, EML, PDF, Gmail, etc. it also suits with the Mac and Windows OS. You can also try the demo and export as many as 20 emails for free.