The jessie-backports repository does not have the latest Iceweasel builds. However, the Debian Mozilla team releases its own backports. To use their backports follow the steps below:
# apt-get install pkg-mozilla-archive-keyring
# echo "deb http://mozilla.debian.net/ jessie-backports iceweasel-release" >> /etc/apt/sources.list
# apt-get install -t jessie-backports iceweasel
At the time of writing this post, the Mozilla team’s repository provides Iceweasel 42, compared with 38.4 with the regular Jessie repository.