Before we try to get the new firefox version working, you should ALWAYS try to upgrade your system through yum if possible. When you manually download something like a new version of software, yum doesn't know that you've done this.
Let's assume that you want to "yum install coolAppVersion4" that depends on Firefox 1.0.4. Yum doesn't know that you have FF 1.0.4 installed, so if it's available, it will re-download it and re-install it. That's best case. Worst case, you will run into dependency problems.
It's been a while since I've used yum, but if you type either "yum update firefox" or "yum install firefox" do you get any joy? The package might also be named "mozilla-firefox" (ie, 'yum install mozilla-firefox'.