Though ???Debian stretch??? is the latest version, it lacks one thing i.e., Firefox Quantum. However, it comes pre-installed with Iceweasel which is an ESR version of Firefox. Today, I’ll demonstrate through step by step instructions on how to download the latest version of Firefox from its official website (Mozilla) and then setting it up as the default web browser on your computer. And succeedingly remove the ESR version of Firefox; if it bothers you.