Download Firefox from this page: https://www.mozilla.org/en-US/firefox/all/#product-desktop-release

Open a terminal and go to the folder where your download has been saved. For example:

cd ~/Downloads

Extract the contents of the downloaded file by typing:

tar xjf firefox-*.tar.bz2

The following commands must be executed as root, or preceded by sudo.

Move the uncompressed Firefox folder to /opt:

mv firefox /opt

Create a symlink to the Firefox executable:

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

Download a copy of the desktop file:

wget https://raw.githubusercontent.com/mozilla/sumo-kb/main/install-firefox-linux/firefox.desktop \
 -P /usr/local/share/applications