If you really NEED to start it NOW then open up a virtual console and type
and it'll start
However, you probably want it in a menu or on your desktop. If you're using KDE, I believe it has some sort of program that will attempt to detect newly installed software and add it to the menu. It's under "system" or "configuration" or something (obviously I'm not using KDE right now or I'd tell you exactly where it is). Gnome may have something similar.
In KDE if you right-click on the desktop and select "new" and then "shortcut" or "application" or something like that you can specify an application to be run. Just type in "firefox" as the application and you should be golden. Then you've got a desktop shortcut. It probably won't have the pretty firefox logo, though
This might help
Sorry if this seems like a little manual work. There's gotta be some RPM out there that might do the menu thing automagically.
-- the dudeman