Mozilla Thunderbird is an open source, cross platform email application that provides you a boatload of email functionalities, such as message management, junk filtering, extensions and themes, and much more. This email client was developed by Mozilla Foundation. Thunderbird keeps your messages, contacts, and settings in a folder named 'Profile' that is present at (for Windows):
C:\Documents and Settings\Administrator\Application Data\Thunderbird\Profiles\wspaeqja.default\Mail'
The messages for a particular folder are stored in MBOX files that carry no extension. If you need to change your email program and move all messages to MS Outlook, then the only option for you is to use a commercial email conversion tool. Stellar MBOX To PST Converter facilitates a smooth migration from Thunderbird to Outlook by allowing you to effortlessly convert MBOX files to Outlook PST files.