Well, there are two ways which i know that can be used to install firefox in a PC. One is a little trick which you do in your machine to fool the OS in believing that only one version of the application is installed. This is how to do it:-

Steps: Mover or rename the installation folder of the existng installation of firefox. Now go to Add Remove Programs and uninstall the application.(it won’t uninstall the application since you have changed or renamed the application).

Now install the new version required. It will install without any problems. I have used this technique many times and it works fine.

The other way is to use a portable version of the application. This version doesn’t need to be installed in the machine. It copies all the required files to the location that you specify and then you can run the program from there itself. Just leave your Program Files\Mozilla Firefox folder alone and install the new version u want to install in a different directory.

You can download the Firefox portable versions from the link below:

http://portableapps.com/apps/internet/firefox_portable
http://portableapps.com/apps/internet/firefox_portable/test