Define key-bindings on a per-window basis.

You can use this module with the following in your xmonad.hs:

 import XMonad.Actions.PerWindowKeys

  ,((0, xK_F2), bindFirst [(className =? "firefox", spawn "dmenu"), (isFloat, withFocused $ windows . W.sink)])

  ,((0, xK_F3), bindAll [(isDialog, kill), (pure True, doSomething)])

If you want an action that will always run, but also want to do something for other queries, you can use bindAll [(query1, action1), ..., (pure True, alwaysDoThisAction)].

Similarly, if you want a default action to be run if all the others failed, you can use bindFirst [(query1, action1), ..., (pure True, doThisIfTheOthersFail)].

For detailed instructions on editing your key bindings, see the tutorial.