yum is an installation manager. It goes off and finds out what's available for you to install and then if you install something it sorts out what other bits and bobs you need to run it and loads them as well.
After you have installed a program it will using the "yum update" command go off and check to see if there have been any updates and if so install them for you.
So say you wanted to have a different movie player you would get to the stage of having a terminal with the # and type "yum install vlc" it would bugger off and sort everything out and install it. And for ever more when you typed "yum upgrade" it would disappear off and get the latest version.
Doing it the manual way will mean that the firefox won't be in the list of applications yum will automatically upgrade.
Anyway glad its working now.