You might be able to read them by using the addon, importexporttools.

''re :This has created a file with file extension mbox.'' Did you rename the eg: Inbox.mbox' file as 'Inbox' and then import the renamed mbox file into the mail account using addon 'ImportExporttools NG' ? Thunderbird uses mbox files but they do not contain an extension. So if you have 'Inbox.mbox' then rename by removing the '''.mbox''' part, so it just says 'Inbox'. You could create a copy first before renaming. Files with .msf extension are indexing files and they do not contain emails. they are used by Thunderbird. Image below shows a Pop mail account folder with typical mbox files with no extension. Although it contains info when using ImportExportTools NG, https://addons.thunderbird.net/En-us/thunderbird/addon/importexporttools-ng/ the point is - it does indicate what are mbox files and how they look. Mbox files can be opened using a text editor eg: Notepad++