Install the newest Firefox from the official website. When you will download it, you will get a compressed folder. Uncompress it and put the folder in system folder /opt. Then you just create a system symbolic link in /usr/bin which has to point to /opt/firefox/firefox (you must preceed the symbolic link command command with sudo because you are messing with a system folder). Now when we will type firefox into the terminal, operating system will reckognize the command ant execute the /opt/firefox/firefox executable file.