You can download eSofttTools PST to MBOX Converter Software if you are searching for a simple way to export emails from Outlook to Mbox. It allows you to simply and smoothly convert all kinds of Outlook PST files to MBOX files. It displays a live preview of emails and attachments during the conversion of Outlook PST files into MBOX files.
For more information:- Export Emails from Outlook to MBOX
Make sure to compact all folders in your Mozilla Thunderbird
Then, download and extract the Gmail Loader
Double-click the file gmlw.exe for launching the Gmail Loader
From Configure Your Email File, go to the Find option
Locate files pertaining to the Thunderbird folder that you need to import into Gmail account. You can find out these files under Mozilla Thunderbird email store folder. Most probably, you will make Windows display all hidden files and folders to view the Application Data folder. Use those files that do not have any file extension (not the .msf files).
Next, click the Open button
Keep in mind that MBOX (Netscape, Mozilla, Thunderbird) file is selected from File Type: in Gmail Loader
If you are exporting sent email messages, select an option Mail I Sent (Goes to Sent Mail) from the Message Type:. Otherwise, choose Mail I Received (Goes to Inbox).
Type your complete Gmail address from Enter Your Gmail Address
Choose to Send to Gmail option