If you want to convert your MBOX files into HTML format to view your emails in any browser, the SysInfo EML File Converter is a reliable and easy tool to do this even though it's primarily designed for EML files, it also supports conversion from MBOX format.

Here’s a simple way to convert MBOX to HTML:

1. Download and install the SysInfo EML File Converter on your Windows or Mac system.
2. Launch the tool and click on “Add File” or “Add Folder” to upload your MBOX files.
3. After uploading, you'll see a preview of all your emails. Select the emails you want to convert and click Next.
4. From the list of saving options, choose “HTML” as your output format.
5. Set your destination path where you want to save the HTML files.
6. (Optional) Use filters like Date Range, Skip Duplicates, or Exclude Attachments if needed.
7. Finally, click on the Convert button to begin the process. Read more: https://www.sysinfotools.com/recovery/mbox-converter.php