Updating firefox in Ubuntu 18.04

Open terminal

Copy the command here and paste and enter
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys A6DCF7707EBC211F


Type password

Then this command
sudo apt-add-repository "deb http://ppa.launchpad.net/ubuntu-mozilla-security/ppa/ubuntu bionic main"
Then
sudo apt update
Then
sudo apt install firefox
OK