Firefox doesn't have a converter for MHT / MHTML format files. You can use Internet Explorer 11, or maybe Edge.

https://www.bing.com/search?q=open+email.mht+files

Thankyou for your reply. It would appear to be more complicated than you would expect. Strange that Mozilla doesn’t have a solution and that you have to go elsewhere for a solution

In the old days when Firefox add-ons could do anything, there were some to read/write .mht files. Firefox 57 limited the powers of add-ons and I am not aware of a new add-on to read/write .mht files. If you don't want to use another browser to read them, you could do a search and see what you find: https://addons.mozilla.org/firefox/extensions/