just use additionalKeysP :)

import XMonad
import XMonad.Hooks.DynamicLog
import XMonad.Hooks.ManageDocks
import XMonad.Util.Run(spawnPipe)
import XMonad.Util.EZConfig(additionalKeysP)
import System.IO
import XMonad.Actions.WindowGo

myManageHook = composeAll
    [ className =? "Gimp"      --> doFloat
    , className =? "Vncviewer" --> doFloat
    --, className =? "mplayer" --> doFloat
    ]
-- instead of the sendMessage composition in the keybinding, you could also use this to "make Master" and "change to layout Full" whenever a window has the "Firefox" classname

main = do
    xmproc <- spawnPipe "xmobar"
    xmodproc <- spawnPipe "xmodmap ~/.Xmodmap"
    fproc <- spawnPipe "firefox"
    termproc <- spawnPipe "lxterminal"
    xmonad $ defaultConfig
        { manageHook = manageDocks <+> myManageHook -- make sure to include myManageHook definition from above
                        <+> manageHook defaultConfig
        , layoutHook = avoidStruts  $  layoutHook defaultConfig
        , logHook = dynamicLogWithPP $ xmobarPP
                        { ppOutput = hPutStrLn xmproc
                        , ppTitle = xmobarColor "green" "" . shorten 50
                        }
        , modMask = mod4Mask     -- Rebind Mod to the Windows key
        , borderWidth = 0
        } `additionalKeysP`
        [ ("C-<Print>", spawn "sleep 0.2; scrot -s")
        , ("<Print>", spawn "scrot")
        , ("M-f", (runOrRaiseMaster "firefox" (className =? "Firefox")) >> (sendMessage $ JumpToLayout "Full"))
        ]