I fired up my old Linux PC which runs on Linux Mint. I noticed the default Firefox browser is quite old. It needs an upgrade, which is expected since this Linux Mint version is 18.1. The current version is 19.1. So, here are the steps to upgrade Firefox in Linux Mint.

Download Firefox.

wget https://www.mozilla.org/en-US/firefox/download/thanks/

Extract.

cd ~/Downloads/
tar xjf firefox-64.0.2.tar.bz2

Move Firefox.

mv firefox /opt/firefox

Create a symbolic link.

ln -s /opt/firefox/firefox /usr/bin/firefox

The old icons should still work. Enjoy.