I recently decided to move from Outlook to Thunderbird.  Finding a way to read those PST files was the main challenge.  Luckily, there’s a free software for it: LibPST.  You’ll have to compile it, but, once you have it working, it’s as simple as “readpst -r -u -D Outlook.pst”  and your mail will magically be transformed into the Mbox format into a folder named “Outlook”.

The next task is to convert that Outlook directory/folder into something that Thunderbird can read.  I used these instructions.  In summary, rename every directory to include a “.sbd” extension, and then move the “mbox” file inside that directory up one directory.  Said another way, open the “Outlook” directory; find the directory named “Inbox”;  rename it to “Inbox.sbd”; open the “Inbox.sbd” directory; rename the “mbox” file to “Inbox”; move the “Inbox” file to the “Outlook” directory.  Now Thunderbird will be able to read your “Inbox” once you’ve added the “Outlook” directory as a special local folder.

Hope this helps!