As an aside, it's the no. 1 reason I don't use Atom: An ugly, slow UI that doesn't render native elements.
So no, it's not indefensible. I think your conspirator's OP comment "As a non-OSX user, I don't care about your UX" is more vain.
Sure it's vain. We all have personal priorities. People suggesting that Mozilla spend time developing the perfect OS X product are expressing their priorities as well.
I could say that I'm a BeOS user and I really think that Firefox should have a fully-integrated native UX because the way a product looks and works is important. But no one cares about BeOS UX because they're vain.
It looks like OS X users are the only ones who really care. Windows users have been dealing with non-standard UIs since tabs first stopped being MDI, and, comparing Chrome's adoption with how it looks, they're mostly fine with them. Linux users are mostly just straight-up crazy and Firefox is fine for them. Smartphone users have seen more non-standard than standard UIs and it doesn't seem to have been a problem for the Facebook app. The question is then is it worth caring as much about OS X UI, or is it more useful to make a better product for the 80+% of users who don't use OS X?
And, I'd argue, Firefox is far, far, far from "technically superior" on any number of axes, regardless.
User experience is important. Dismissing that aspect as "animations [that] aren't as nice" misses an important point - things like animation, consistency with the host system, integration with platform features and so on are really very important to the quality of an app.
Technical capability is also important, obviously. It's why I use Firefox and when building web projects on the Mac - best feature set. But I use Safari instead for day-to-day browsing, because of things like better scrolling and zooming, and better integration with platform services.
I do think the way a product looks and works is important, and developers who dismiss that are why we have so many awful UIs out in public.
I agree user experience is important, but experience is so much more than simple aesthetics.
I'm not saying they're totally unimportant, which is why I included the qualifying clause about "undue hardship", but I have no respect for users that will put themselves at a functional disadvantage so that their experience can be "prettier" (aka "more native"). These people have their subculture, called "Apple", and frankly, I want as little to do with it as possible. Prioritizing glitz above function shows serious problems in critical thinking.
If we can make an application look better, cool, we should add that to the list somewhere. I'm not opposed to that. I strongly disagree that this should be anywhere near the primary criterion used to judge an application's value.
Edited to add: And I disagree with them, however I'm downvoting to show disagreement is something that HN has always been against, to it's credit.
I'll tell you what I can't comprehend – people who apparently find it difficult to accept that other people have differing priorities to them, and aren't mentally subnormal as a result.