The state of mhtml support is fairly pathetic at the moment. Firefox broke mhtml compatibility with the quantum overhaul. Chrome's mht support had been a hit and miss over the years, sometimes removing the GUI option entirely and requiring one to manually launch the browser with a special tag to enable it. The only browser with a history of consistent mhtml support happens to be....Internet Explorer, followed by a bunch of even more obscure vendors that nobody really uses.

I am currently dealing with the problem of parsing large mht files (several megabytes and up). A regular web browser would hang and crash upon opening these files and most ready made tools I could find struggle with the number of embedded images. It's very much a neglected format with very little support in 2019.

According to the MHTML entry on Wikipedia, Chrome requires an extension, Firefox doesn't support it, and only Internet Explorer supports MHTML.

I mean, it's not any worse than WARC support…

Maybe MAFF's are best as they use compression instead of base64 encoding: https://en.wikipedia.org/wiki/Mozilla_Archive_Format

Or SingleFileZ files which can be viewed without installing any extension https://github.com/gildas-lormeau/SingleFileZ.

Edit: it can also auto-save pages, like SingleFile.

I used the excellent unMHT plugin for Firefox, but it got dropped some time ago, failed to meet "enhanced security requirements" :(

I still keep an old ESR with this plugin for archiving, and accessing MHTs.