Upgrade to Iceweasel latest version /Firefox latest version on Debian Wheezy

Add these two to your /etc/apt/source.list

deb http://backports.debian.org/debian-backports wheezy-backports main
deb http://mozilla.debian.net/ wheezy-backports iceweasel-release

aptitude update
apt-get install -t wheezy-backports iceweasel