"I suggest you try debugging" is never a (complete) valid answer to any StackOverflow question. As suggested by the other answerers, if a question is so vague that the only thing an answerer can think of is "use firebug" then it means that there is not enough information to answer it.
However, there are often questions which do have valid answers, and the poster is still told to simply use firebug. This is NOT the behaviour we should be encouraging. Any teacher will tell you that the answer alone will not usually help anyone.
To use the standard analogy. If you give a starving man a fishing pole, well, he still can't eat that.
The best response is both. An answer along the lines of
"In Firebug (link) there is a 'Something' panel. If you use that, you'll notice that XYZ is causing your problem of ABC. To resolve it, you could foo the bar. [Code sample of you fooing a bar]"
That way you have both solved their problem, and grown them as a programmer.
Answer the question AND encourage them to use firebug.
- It is not accusatory and condescending the way "Use Firebug" is.
- It encourages them to solve their own problems in the future by giving them the tools.
- It solves their problem directly, which encourages them to return to StackOverflow in the future.