Download firefox from https://www.mozilla.org/en-US/firefox/
unpack and copy into /opt folder
sudo tar xjf ~Download/firefox-57.0.4.tar.bz2 -C /opt/
remove downloaded package
rm ~Download/firefox-57.0.4.tar.bz2
remove old firefox
sudo rm /usr/bin/firefox
symlink new firefox into /usr/bin folder
sudo ln -s /opt/firefox/firefox /usr/bin/firefox
Thanks to: https://askubuntu.com/questions/25961/how-do-i-install-a-tar-gz-or-tar-bz2-file