Mozilla Firefox is a default Internet browser on Ubuntu 18.04 so this article only shortly mentions installation and focuses also on uninstall and update processes.
The article suggests linking the firefox executable only, but that can cause problems.
I would suggest the following:
# remove the current version of firefox (don't move it)
sudo apt-get remove firefox
# fetch and extract
wget -O ~/FirefoxSetup.tar.bz2 "https://download.mozilla.org/?product=firefox-latest&os=linux64"
sudo tar xjf ~/FirefoxSetup.tar.bz2 -C /opt/
rm ~/FirefoxSetup.tar.bz2 # cleanup
# remove any lingering remnants of the old firefox
sudo rm -rf /usr/lib/firefox
# link the entire directory for firefox so that policies or autoconfigs can make it in safely
sudo ln -s /opt/firefox/ /usr/lib/firefox
# recreate the link that the terminal would expect
sudo ln -s /usr/lib/firefox/firefox /usr/bin/firefox
# test it out
firefox --version