Updated 01-Oct-2023
Firefox has dramatically disintegrated under the so-called leadership of Mitchell Baker, enriching herself to the tune of millions while the market share (and revenue) of Firefox plummeted and 250 employees were laid off. This alone is reason enough to avoid such a toxic software environment. There is no need to coddle multi-millionaires in order to save open-source projects.
That said, for those who want to install a normal Firefox build (and not an ESR), and isn't a fan of Snaps or Flatpaks, the following incantations are necessary (another technical failure):
First, to remove Firefox ESR:
sudo apt purge firefox-esr
Then to install stable Firefox:
sudo mkdir /etc/apt/keyrings/
sudo gpg --keyserver keyserver.ubuntu.com --recv-keys 2667CA5C
sudo gpg -ao ~/ubuntuzilla.gpg --export 2667CA5C
cat ubuntuzilla.gpg | sudo gpg --dearmor -o /etc/apt/keyrings/ubuntuzilla.gpg
Wait for confirmation
sudo rm ~/ubuntuzilla.gpg
echo "deb [signed-by=/etc/apt/keyrings/ubuntuzilla.gpg] http://downloads.sourceforge.net/project/ubuntuzilla/mozilla/apt all main" | sudo tee /etc/apt/sources.list.d/ubuntuzilla.list > /dev/null
sudo apt update
sudo apt install firefox-mozilla-build
To remove Firefox stable
sudo apt purge firefox-mozilla-build