This extension does not make Firefox into a MHT reader, unfortunately. MHT files still can't be displayed.

We need such a plugin (on Windows) - I have zillions of old MHT files that I won't bother converting, especially since the format isn't dead in Chrome or Android. I keep an old version of Waterfox around just to read MHT (and remember my favorite FF plugins). With SingleFile and Save Page WE around nobody needs to WRITE mht anymore, but it's just lame Firefox is missing the capacity to read them.