I bought a Macbook Pro yesterday and installed firefox. I used firefox a few times, but now I can't find it. How can I find where it is?
Something else that might have happened is that you ran Firefox directly from the DMG instead of actually "installing" it.
Firefox comes in what is called a DMG. DMGs are basically virtual disks that hold data, and then when you double-click them they essentially "plug in" to the system, and look just like another drive. What you may have done is run Firefox directly from the DMG, instead of "installing" Firefox. Then, perhaps after a reboot, the DMG got unmounted (essentially "unplugged", no longer looking like a drive) and so you couldn't find the icon any more. It's an easy mistake to make.
To actually "install" something from a DMG in Mac is really easy. Here's how.
In the upper right hand corner, there is a magnifying glass. That is the spotlight search feature of OS X. Click it and type in 'firefox' and press enter and it will show you where anything to do with 'firefox' is. It would likely be under the Applications folder, but it could be put anywhere and ran.