But I want HN code snippets to look like code because sometimes they're used for code.

let's not go back on works-on-IE-5 era. sites should work on the browsers that people use, not the other way around. i don't think it's fair that you got downvoted, though, your intention was good, you provided a solution.

This does not appear to be true on iOS? If it is...how?

That's because Firefox for iOS is really just a skin around Safari's WebKit-based engine, because Apple plays the evil gatekeeper on their app store and doesn't allow Firefox to use their own rendering engine. (Chrome on iOS is also effectively just a Safari wrapper.)

It's one of the many reasons I think iOS is a horrible choice for a privacy-conscious user.

Firefox for Android supports extensions.