I want to download firefox browser for my macbook air 13", but cannot find it on the mac store and when downloaded from firefox my mac says cannot install because cannot verify author (something like that). Are mac and Firefox in a fight.
You can install it by right clicking and selecting the open option.
By default, OS X has GateKeeper enabled which is, essentially, a piece of software that limits what can get installed to those pieces of software developed by programmers registered with Apple. You can disable this in Security (System Preferences) but I'd recommend keeping it enabled if you're new to the platform.