How to open MHT files on Firefox 58?

To display MHT files:

• Use IE or Chrome, since they both support the filetype natively, or
• Use an outdated version of FF that still supports the UnMHT extension.

To migrate MHT files (e.g., to be displayed in modern FF):

• After displaying the MHT file in a browser, re-save the page as "Web Page, Complete", and optionally serve converted MHT files over localhost with a web sever (e.g., Python 2's SimpleHTTPServer) to FF so Save Page WE can access and convert them, or
• Use MAF on an outdated version of FF to bulk convert MHT files into another format.

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Needless to say, outdated versions of web browsers are unsupported and have known unpatched security vulnerabilities. For instance, Firefox 58 is forever vulnerable to the Pwn2Own exploit that was publically demonstrated back in March 2018.

Additionally, Mozilla will be deleting legacy add-ons that rely on XUL/XPCOM technologies from AMO, so the downloads for MAF and UnMHT will disappear from that site in the future.