Firefox doesn't come as source when you download the tar file. It's precompiled. The best way to go about integrating it with your OS is to do the following as root (if you only have sudo commands then you can log into root by "sudo su -").
mv firefox-version.tar /opt/
tar -xvf firefox-version.tar
ln -s /opt/firefox/firefox /usr/bin/
------- Other recommendations -------
After doing all of that I usually move the /opt/firefox folder to it's version and then link it to /opt/firefox like so...
mv firefox firefox-10.0.0
ln -s firefox-10.0.0 firefox
This way when you update you can simply remove the symlink, untar the new version of firefox to /opt, mv /opt/firefox to it's updated version, then resymlink firefox again to /opt/firefox. This way you can have multiple versions of firefox installed and the only symlink which needs to be changed is /opt/firefox.