This question has been asked before, but I have a different problem and the posted one is way outdated.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.
I solved this problem. While I did get a profile selection box and did create a new profile (firefox54), I did not notice that "choose this as default without asking" was automatically checked. I started all over again and unchecked it and it worked.
I just have to be careful when I load the new firefox to choose a different profile. Otherwise, it will see the plugins and remove them.
Picture below