The second map packs (if there is going to be one at all) will most likely be released sometime in July, as Modern Warfare 2's 2nd map pack came out in July, and they tend to follow older games.
I don't know about that zombie map though, it looks kind of sketchy, and I doubt "Firing Range" will be the name of the 2nd map pack. Since that sounds more like a name that would fit a single map.
For right now, I'm still getting use to First Strike, so I hope they don't come out with a new map pack for a while longer, so I can get my full fun with First Strike.