You can do it, and without rebuilding from hacked source, but it's more work than install $anythingelse, use once, uninstall.

When I searched "firefox bypass hsts" I got a years-long thread where a mozilla dev said how to do it.

Just finding and merely reading that post was more work than install foobrowser, search how to use it's bypass method, use once, uninstall.

Since it's possible, seems ripe for a plugin? Is that something plugins can touch? IDK, as before the nuclear option is actually more convenient. Meanwhile the browsers with nerfed plugins actually empower the user more at least on this point.

Proves that monoculture is bad even if it's your favorite that wins. If it was FF that was taking over everything instead of chromium, then it would be chromium proving that monoculture is bad.