How to Download old version of Firefox
If you want to download old version of Mozilla Firefox for testing purpose or any other reason Mozilla have This Website which enable you to download Mozilla Firefox old Versions.
Downgrade Firefox is a Security Risk
Notice that it is not recommended downgrading Firefox and going to older version, doing so you make your computer vulnerable to security threats and attacks. It is always recommended to install latest version of Firefox Web Browser and Update the browser to the latest version all the time.
Anyway, just go to https://ftp.mozilla.org/pub/mozilla.org/firefox/releases/ and mozilla firefox old version available to Download for you. All the way back to 2004.