Paste the below commnads one after the other , or paste it in a file with 

#! /bin/bash as the starting line { save it as ff and chmod 777} and say    ./ff
  Download from : mozilla

 sudo tar -jxvf  firefox-3.5.tar.bz2 -C /opt

 sudo mv /opt/firefox/plugins /opt/firefox/plugins.bak
 sudo ln -s /usr/lib/firefox/plugins /opt/firefox/plugins

 sudo rm /usr/lib/firefox/plugins/libtotem_mozilla.*

 sudo dpkg-divert –divert /usr/bin/firefox.ubuntu –rename /usr/bin/firefox

 sudo ln -s /opt/firefox/firefox /usr/bin/firefox

 sudo chown -R root:root /opt/firefox

 firefox&
Thats it :)   



Or just use the hack by Kevin Purdy to install FireFox :

wget -O - http://releases.mozilla.org/pub/mozilla.org/firefox/releases/3.5/linux-i686/en-US/firefox-3.5.tar.bz2 | tar xj -C ~

More info :

downloadstats Worldwide Real-Time Firefox Downloads .

Sneak Peek: What’s On Tap for Firefox in 2010 Firefox 3.6