To install Iceweasel 3 you need to use Backports package archives.

Add the following to: /etc/apt.d/sources.list

deb http://www.backports.org/debian lenny-backports main contrib non-free

Then as root run apt-get update

To install packages from backports apt-get -t backports install <package name>

eg. for iceweasel, as root

$ apt-get -t backports install iceweasel