This question already has an answer here:
This question has been asked before, but I have a different problem and the posted one is way outdated.
The problem is that I need an old and new firefox running at the same time to run a plugin called "Digital Pali Reader" which will not work with the newer firefox.
I followed the instructions from the website https://ubuntuforums.org/showthread.php?t=1907045 and downloaded from: https://ftp.mozilla.org/pub/firefox/releases/54.0/linux-x86_64/en-US/
It worked like a charm until I ran the new firefox. Then it disabled the plugins which was the goal of running the old version.