What is the easiest way to convert PST to MBOX for Thunderbird?

I have some Outlook PST files containing important emails and want to move them to Mozilla Thunderbird. Since Thunderbird uses the MBOX format, I'm looking for a reliable way to convert PST files without losing email data, attachments, or folder structure. Is there any simple method or tool that can help me convert PST to MBOX Thunderbird compatible files? I'd also like to know if the process works with large PST files.