How to export messages from Outlook 2003 to ThunderBird?
How to import messages to ThunderBird from Outlook 2003?
From my last experience I can say that the easiest way is to import/export this way:
Outlook 2003 -> Outlook Express – > Thunderbird
Next step could be using MozBackup application to make backup (export) database, eg for deploying on other machine.
Related links:
How to export messages from Outlook 2003 to Gmail?
Just use dedicated Google Application: Google Apps Migration for Microsoft Outlook
From my exprience – export of 250 000 messages took 7 days (no matter of the Internet connection speed).