[Springtime]

They used to support MHTML prior to the Quantum update, via the Mozilla Archive Format addon or the superior UnMHT addon (which captured pages more accurately than Chromium's MHTML support did in direct comparisons I made).

Not sure why they dropped support for it entirely since, given it was supported for the longest time and it's the most convenient single file format for web page saving. It's a major reason I couldn't continue using Firefox as my main browser.