There are some manual steps by which you can easily import MBOX to Thunderbird mail program.
Steps to Open/Import MBOX File in Thunderbird:
Step 1: Open Mozilla Thunderbird.
Step 2: Go to » Add-ons
Step 3: Click on Extensions. Search Import Export Tool.
Step 4: In Add-ons Windows, check ImportExportTools and click on "Add to Thunderbird".
After this, you will get a dialog box with the installation option of ImportExportTools.
Step 5: Click on "Install Now" button to start the installation process.
Step 6: Restart the Mozilla Thunderbird. Click on "Local Folders" to import MBOX file there.
Step 7: Go to » Tools » ImportExportTools » Import MBOX File.
Step 8: With "Import directly one or more mbox files" option selected, click Ok.
Step 9: Select MBOX file and click on "Open" button.
Step 10: Your MBOX file will instantly appear in Local Folders. Just click on MBOX file and view in preview pane.
These are the simple steps to import MBOX file in Mozilla Thunderbird. If you have still any doubt or question, feel free to ask.