You have received 3 upvotes (4 now, if you include mine :) and 0 downvotes on that answer and also had another user defend it from the one who was objecting to your answer being only for *nix. I don't see what the problem is.
Remember that SU has 199k registered users. Of these, perhaps 5 (3 votes + 2 commenters assuming neither of the latter voted) have taken an interest in your question. Of these at least 3 have considered your answer good, one of them or perhaps another user defended the specific nature of your answer and one and relatively low rep user asked you to clarify that it is *nix specific.
I don't see how you can conclude that SU wants OS-independent answers. If anything, it looks to me like your answer was well received, exactly as it should have been.
In any case, it is a good idea to explicitly mention if an answer is platform-dependent. It avoids this kind of issue and is helpful for everyone. I understand why you left the question vague, that way it can be more useful if other users answer for different OSs, that's fine (not the case in your firefox question by the way since the issue itself is Linux specific). Answers are different though and it is a good idea to clarify that if they only work on a specific OS. Especially when that OS isn't Windows since many Windows users are not even aware of the existence of other operating systems.