Is it really so difficult to add native support for mht, maff and similar files?
I think the demand for those formats is just to low. Many modern pages either work offline out of the box, can’t be saved at all or can be saved as PDF of screenshot.
As for the UnMHT extension (whiteout knowing it) I think you have to use some UI element of the extension to open the file and cant just “open it with Firefox”. AFAIK WebExtensions can’t install file format handlers, i.e. Firefox doesn’t know to forward the file to the extension.