Text is a difficult medium to gauge emotions from. I've written plenty of condescending posts that were carefully worded to avoid sounding condescending, and I've written plenty of friendly posts that accidentally appear condescending. In general, I'm a pretty condescending person (even in face-to-face conversations), and I have to try hard to not be sarcastic or arrogant - but it still leaks through. Luckily, I'm a very helpful person also (online and face-to-face), so people are usually willing to overlook my condescension while benefiting from the help, but that still gives me plenty of room for character growth.
While I don't think the 'Firefox' comment is helpful, many questions do have good answers on Google, where creating yet another thread wastes everyone's time, including the poster, since they would get their answer faster if they just searched the web. But I don't use the lmgtfy site, because that does seem condescending to me, but pointing out that someone needs to do basic research is, I feel, a necessary criticism people occasionally need to hear.
That being said, some questions where people ask and never show up again still have value to other members who lurk around. I've found threads on the internet where the OP never responds, but since the answer was given, it answers *my* question from googling. I've also participated in threads here on GameDev where the OP doesn't respond, or responds with "That solved my problem, thanks!", but the thread continues onward without them while continuing to provide value to the people who are discussing it.
I've also several times have asked questions and failed to respond to the answers. 
In rare situations, I don't feel motivated to respond - unsure of what to so, or capable of responding gracefully without biting someone's head off. I do try to respond to almost every thread I start. Other times, I'm waiting for more answers before replying, and the more answers don't arrive, and it feels silly to say, "Your posts didn't answer my question but thanks for responding guys" two weeks later, bumping the thread unnecessarily. Not responding and not returning aren't always the same. It's possible for OP's to read the answers, benefit from it, but for some reason or another, not feel comfortable responding or don't see a need to respond.
It'd be nice if they respond, but even if they don't, the thread isn't an entire waste. As much as I love heartfelt kudos from people who I've helped, that's not the main reason why I post.
Personally, I'd rather have someone ask a question and not respond to my answer, and happily assume they've figured it out, then to answer someone's question, and continue to invest several hours of back and forth posting, with the final result being that they don't actually want to learn and just want someone else to solve the problem. That can be draining to me, because once I start engaging in conversation, I feel obligated to continue, and if they aren't actually gaining value from my posts, it just makes me grumpy. 
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