I am using ubuntu 8.04 because of my hardware restrictions.
I followed the steps to install latest firefox as i can use but i get firefox 3.6 could not found response:

For Ubuntu 8.04 (Hardy) Users

deb http://ppa.launchpad.net/ubuntu-mozilla-daily/ppa/ubuntu hardy main

deb-src http://ppa.launchpad.net/ubuntu-mozilla-daily/ppa/ubuntu hardy main

Save and exit the file

Now you need to add PPA GPG key

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 247510BE

Update the source list

sudo apt-get update

Install Firefox 3.6

sudo apt-get install firefox-3.6

If you already have a version of Firefox 3.5 installed from a repo
then upgrade using the following command

sudo apt-get upgrade