There are several places you can do this.
1:
go to your Activities overview, select Applications. You should see the "Add/Remove software icon near the top of the list. (It has an icon that looks like 4 boxes)
2:
open a terminal window, or use ctrl-alt-f2 to open a console
if in a terminal, IF in a console, jusr login as root and run
yum install firefox
Code:
su -
(root password)
yum install firefox