I think it's that your response sounded like a lazy dismissal. Reasonable defaults are good, yes. That's pretty uncontroversial. But nobody has established that Firefox's behavior is less reasonable than the alternative — for example, the comment you were replying to noted that both approaches have their fans — so your out-of-hand rejection on behalf of "users-on-street" sounded imperious.

The only reason I didn't question the behaviour is that the parent didn't either: they just mentioned that most users could customise. Which, as mentioned, they wouldn't want to do.

Noticing the folk in the conversation there's lots of newish accounts, and some are replying before they've actually read what they're replying to. Downmods for 'I disagree' are more popular now as well.