If you are like me, you like to use the latest plugins for Firefox (when you use Firefox). Things like Firebug have become a staple for my web browser usage. I also love using Debian as my OS, and the stable version comes with a much older version of Firefox (sans logos and with the name iceweasel). Unless you download older version of plugins, most of the time you'll have trouble installing the plugins you want.
The simple solution is to download and install the latest version of Firefox directly from Mozilla. There's not much of an install process. Just download and uncompress the file. You'll get a firefox directory, which I moved over to my home directory and proceeded to create shortcuts for on my desktop and menus.
If you are using the 64 bit version of the OS, you will quickly run into the "Couldn't Load XPCOM" error. It's obvious that it is a library issue but the error doesn't give you a big clue as to how to fix it.
Good news, the fix is extremely easy. You are lacking some 32 bit libs that are required to run the program.
Fix is by running the following as root or with sudo:
apt-get install ia32-libs-gtk
After that, you should be able to run Firefox. Enjoy!