I am unable to open MBOX files in modern email clients like Outlook or Thunderbird: What can I do to preserve their contents?

I recently found some old MBOX email files that were created years ago using an email client that is no longer available.

Unfortunately, I am unable to open these MBOX files in modern email clients like Outlook or Thunderbird. I am worried about losing important emails and attachments. Is there any working and reliable solution to open or convert old MBOX files without data loss?