Iceweasel is no more at Debian.
More details about this decision : https://wiki.debian.org/Iceweasel
A new repository must be enabled : jessie-backports managed by The Debian Mozilla team
Add a new sources.list file in /etc/apt/sources.list.d/ with the following line :
deb http://mozilla.debian.net/ jessie-backports firefox-esr
Run the following commands to have Firefox installed on your system :
[fool@localhost]$ sudo apt-get update
[fool@localhost]$ sudo apt-get install pkg-mozilla-archive-keyring
[fool@localhost]$ sudo apt-get -t jessie-backports install firefox-esr
Firefox is now installed. Enjoy !