Unfortunately the site only support most recent Browser and anything older is consider security risk to Firefox. What your looking for to be done will have to look online to do what your asking but not on the Firefox forum.

''<p>joao.osvaldo [[#question-1412485|said]]</p>'' <blockquote>I need an older version of Firefox (version 50) to enter a specific website, but everytime I close and open Firefox again he updates alone. How can I prevent it from updating? </blockquote> Is it specifically Firefox 50.0 or older you need? You could try portable Firefox 50.0 or even 52.9.0esr https://sourceforge.net/projects/portableapps/files/Mozilla%20Firefox,%20Portable%20Ed./Mozilla%20Firefox,%20Portable%20Edition%2050.0/ https://portableapps.com/apps/internet/firefox-portable-legacy-52 The portable Firefox is a long running project that uses the official builds of Firefox with a custom launcher that keeps things self contained as you can even run it completely off a usb flash drive if you want. It will not tough your regular Firefox release install or Profiles for it.