I don't think it was ever native in Firefox, there is/was the excellent unMHT extension that was broken by Quantum/WebExtensions and The Great XUL Silliness. Shame.

I have Waterfox-Classic and unMHT (fished out of the Classic Addons Archive, just remember to turn off Waterfox's multiprocess feature) since I occasionally need to archive web pages - and more importantly, reopen them later.

mhtml is just MIME, literally every discrete URL as a MIME part with its origin in a Content-Location header, all wrapped in a multipart container. I don't understand why it's not a default format.

I can see WebExtensions breaking it (as it's a completely new set of APIs for extensions, and the losses do definitely still hurt)... but quantum/xul? How is that related, aside from "it happened around the same time"?

IANA firefox dev: XUL/XPCOM = old APIs, WebExtensions = new (multi-browser) API

Quantum was the the project name to re-engineer Firefox internals, with lots of design changes, not just plugins. XUL/XPCOM APIs were dropped, as an occasional programmer I understand why, "Quantum broke my plugins" is a reasonable first approximation for most users.

Safari supports webarchive, which does basically the same thing

The problem is that it is a proprietary format. The advantage of the format produced by SingleFile (HTML) is that as long as your browser is capable of interpreting HTML, you will be able to read your archives without worries.

Not so proprietary. It's really just a plist file, which the format is known and even open sourced by Apple[1]. Really it's only proprietary in that no other platforms have implemented it.

[1]: https://opensource.apple.com/source/CF/CF-550/CFBinaryPList....