First, you need to import PST files from Outlook and then convert the imported PST files into MBOX format using the Thunderbird application. Below mentioned are the steps to convert PST to MBOX for free:

1. Start the Outlook Application and import the PST files.
2. Now, run the Thunderbird Application and click Tools>>Import option.
3. In the Import wizard window, click on Mail and tap on Next.
4. Now, select Outlook from the list, and go to the Local folders.
5. Here, you will find the Outlook Import folder containing all Outlook PST files. Click Close.
6. Now, launch Thunderbird and install the Import/ExportTools NG add-on.
7.  After that, go to the Tools and click on the Import/ExportTools.
8. Export all the folders and select the location to save the file.
9. Finally, all the PST files are converted to MBOX file format.

However, performing this method requires a lot of time and effort. There is also a high chance of data loss. Therefore, many experts prefer using the professional PST Converter to make the conversion process easy. The tool offers a free demo version that can convert up to 50 emails per folder at no cost.