Keeping old version of Firefox while updating to new version
Older versions of Firefox can be found here:

https://ftp.mozilla.org/pub/mozilla.org/firefox/releases/

There's Multifirefox app, which can help switching profiles for several versions.

Installation:

brew cask install multifirefox



  MultiFirefox is a small launcher utility that allows you to run multiple versions of Firefox side-by-side.  It helps you set up multiple profiles (one or more for each version of Firefox), remembers your last launch preferences for easy launching, has auto-updating built in for when we make improvements, and is 100% open source, MIT licensed.