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Generally speaking in Linux, we get our "packages" (applications, system libraries, the kernel, etc.) through something called a repository or "repo" that is basically a collection of software that's tested and trusted to work with that distro. Each distribution or "distro" maintains its own repo, and the CentOS repo is widely regarded as one of the most stable and well tested repositories out there.
To install an application from the CentOS repo, all you need to do is 'yum install firefox'. Try it; you should see a message that Firefox is already installed. To keep your packages updated with the latest from the CentOS repo, type 'yum update'. Use these two easy commands (or type 'man yum' if you need more options), and you will have a stable and successful CentOS experience.
Now this leads to an obvious question: "What if the application I need (VLC) is not in the CentOS repo, or I want a newer version than CentOS provides (Firefox)? This is a pretty vast question that I cannot comprehensively cover, so let me look only at the specific examples of VLC and Firefox:
VLC is not in the CentOS repository. So I went to http://videolan.org
for you and looked at their instructions for installing it in Red Hat Enterprise Linux (same as CentOS): http://www.videolan.org/vlc/download-redhat.html
They recommend adding an extra repository called RPM Forge that contains additional software that is unsupported by CentOS. You will have to decide for yourself whether you trust this repository and want to use their software. Here is some reading material:
Now as for Firefox, again as I mentioned above, I recommend simply using the stable, tested, trusted, and security-patched Firefox from the CentOS repo. But if there is some new feature you absolutely need to have, I suppose you could download the .tar straight from Mozilla.org. If I remember correctly, all you need to do is extract the .tar (using your favorite Archive Manager) and browse around with your file manager. There is a Firefox "executable" that you can launch by double-clicking (just like a Windows .exe). You can create a shortcut to this executable directly on your desktop if you wish. Please note that if you go this route, YUM will not know about it, so you will have two versions of Firefox installed unless you use YUM to remove the old one. (And if you want the new Firefox available to all users on the system, you can move it to /opt as mentioned above.)