How to downgrade to Firefox 56.0

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.… Read the rest