Comment 123 for bug 240133

RFC 2557 MHTML is a standarized format for web page archive that deemed to be directly readable for web browsers and is already supported by many browsers such as IE and Chrome.

However, Firefox does not directly support it. Although we currently have addons like MAF (https://addons.mozilla.org/zh-tw/firefox/addon/mozilla-archive-format/) or UnMHT (https://addons.mozilla.org/zh-tw/firefox/addon/unmht/), they are written in XUL/XPCOM, which is going to be deprecated, and their functionality is currently not available in WebExtension, the exclusively addon system in the future.

Therefore, we need a support for RFC 2557 MHTML support, at least a directly reading, and saving at best. This could either be:
1. Directly support for MHTML reading and writing
2. Add enough API to allow an addon like MAF or UnMHT in WebExtension