In a former post I've installed iceweasel 4 from http://mozilla.debian.net/ ( version 4.0-1~bpo60+1 ).
To upgrade to the version of experimental I've added this line to /etc/apt/sources.list :
deb http://ftp.nluug.nl/pub/os/Linux/distr/debian/ experimental main contrib non-free
As root :
apt-get install -t experimental iceweasel
Iceweasel was upgraded to version 4.0-3.