1. Unpack the archive with tar zxvf firefox-installer<something>.tar.gz
2. cd firefox-installer
3. Then you will run the setup script (I don't remember what it is called--check the documentation)
It might be a command like ./firefox-installer
After it installs the executable bit will be in the firefox-installer folder and called firefox. To link that to your path just do a :
ln -s /home/you/firefox-installer/firefox /usr/bin/firefox
Now, just type firefox from the commandline and firefox will run. You can make your own icons, add an entry to your menu or whatever...