Backup your current settings if needed
cp -rp .mozilla/ .mozilla_bk
Purge current firefox
sudo apt-get purge firefox -y
Download 56 version
wget https://ftp.mozilla.org/pub/firefox/releases/56.0/linux-x86_64/en-US/firefox-56.0.tar.bz2 sudo su tar -xvf firefox-56.0.tar.bz2 cp -rp firefox /opt/ cd /usr/bin ln -s /opt/firefox/firefox firefox
Finally, you have to avoid upgrading to the newer version.
apt-mark hold firefox
Disable also auto update from the inside firefox settings
about:config
app.update.auto - false app.update.enabled - false app.update.silent - false
Done