Downgrade FireFox

apt-get remove firefox
mkdir fire
cd fire
wget http://releases.mozilla.org/pub/firefox/releases/39.0b7/linux-x86_64/en-US/firefox-39.0b7.tar.bz2
tar xvf firefox-*.tar*
mv firefox/ /opt/firefox3
mv /usr/bin/firefox /usr/bin/firefox-old
ln -s /opt/firefox3/firefox /usr/bin/firefox