Edit /etc/apt/sources.list
deb http://backports.debian.org/debian-backports squeeze-backports main
deb http://mozilla.debian.net/ squeeze-backports iceweasel-release
Install the GPG key for Mozilla
wget -O- -q http://mozilla.debian.net/archive.asc | apt-key add -
Update package list
apt-get update
Install latest iceweasel
apt-get install -t squeeze-backports iceweasel