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Import Outlook PST to Thunderbird
If you are running Outlook and Thunderbird on the same machine, then you can easily import Outlook mails to Thunderbird. Just follow the steps given below:

Step 1: Make Outlook your default program for sending/receiving emails. Go to the Start button and click Default Programs >> Set your default programs.
Step 2: Select Outlook from the Programs tab and click Set this program as default. Then click OK to save the changes.
Step 3: Now, launch Thunderbird and click on the Display the Thunderbird Menu option.
Step 4: Go to Tools >> Import.
Step 5: Select Mail, or you can select the Import Everything option and click Next.
Step 6: Select Outlook as the file type and click Next.
Step 7: In the last window, click on the Finish button to complete the import process.


Use Google Take Out to export Outlook PST to MBOX

Step 1: Open your Gmail account and go to Settings >> Forwarding and POP/IMAP and select the radio button “Enable IMAP.”
Step 2: Launch Outlook and go to File >> Add Account option.
Step 3: Enter the login credentials of your Gmail account and hit Enter.
Step 4: Create a new folder within the newly configured Gmail ID and move the desired Outlook emails to that folder.
Step 5: Now, Log in to your Gmail account using Chrome or any other efficient browser and open Google Takeout
Step 6: From the Select data to include tab, deselect all options, and check the Mail option only. Also, make sure to check the Include all message in Mail option provided under the All Mail data included category. Click Next after making the necessary changes.
Step 7: Now, in the Customize archive format tab, specify the Delivery method, Export type, File type & size, and hit the Create archive button.
Step 8: Once your archive is generated by Google, you can download the ZIP file containing the Outlook data in the MBOX file format. Click Download to download the ZIP file in your local machine.
Step 9: Now, extract the downloaded ZIP file in a convenient directory. You will get the MBOX file containing all your emails, including spam and trash.
Step 10: At last, import the downloaded MBOX file using the Thunderbird email client.

Hope this helps you out.
Regards,
Jake Paul.