After downloading firefox 3.6 it has no instructions how to install it. When I google for it, the top pages tell you about installing through apt adding the ppa sources for firefox
But all this is not necessary, you can uncompress the firefox download in your home (for example in ~/home/bin/firefox3.6) and double click in firefox file. This is a shell script that run firefox from the current working dir (it invokes run-mozilla.sh). But be sure that you have quit your older firefox. If not when you double click on the firefox file, it will open a new window for your older firefox.