description of your problem to get a good answer. You might have bushing wear in the fork, which causes a bit of a loose feeling, but is not adjustable (you can have the bushings replaced, however). You might have a loose headset, which you can adjust fairly easily. You might have some bearing play developing in your hub, which might be adjustable, depends on type of hub. I assume your quick release is properly tightened, but that could be it too. Try looking at Park Tool's website, there's a lot of good tool and maintenance information there.
I just read your question again...you mean firm up
your front fork? Depends what kind of fork you've got, you might be able to adjust it externally maybe internally; increasing your preload might work, increased damping might help. You might need stiffer springs or elastomers or pump more air in it, depending on the type of fork you've got.
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