Iceweasel 5 is already available for Squeeze users via mozilla.debian.net (instructions are also on that page). As for Lenny, it looks like there's only a backport for Iceweasel 3.6 available, so you may have to consider updating to Squeeze if you want the most current release of Iceweasel. Hope this helps! Kind regards, - Vincent
Actually, today iceweasel-release from m.d.n is installing version 6.