Also, Debian uses the ESR (Extended Service Release) version of Firefox for their current distro's. So currently if you do
apt-get install iceweasel
you will get v38.3.0, not 41.0.2 which is the latest. In order to get the absolute latest firefox, you'd need to install the binary version of actual Firefox (which isn't difficult), or run testing/sid and use the experimental repository which is not suggested for beginners, or anyone that doesn't know how to repair their system when updates break multiple programs, or sometimes even the entire desktop.