For Ubuntu 10.04.

sudo add-apt-repository ppa:mozillateam/firefox-stable

sudo apt-get update

sudo apt-get install firefox

From : http://support.mozilla.com/en-US/questions/795196

If behind firewall,
$ sudo bash
# export https_proxy=http://www.example.com:8000/
# add-apt-repository ppa:

From: https://bugs.launchpad.net/ubuntu/+source/software-properties/+bug/516032