Is it possible -without issues- to have a regular installed Firefox instance and a portable Firefox too, on the same Windows instance? tl;dr I want to use Firefox as normal, and that's no issue, but now -that Quantum is so different- I also want to have a pre-Quantum Firefox (56.0.2) available for one specific site with one specific add-on that is no longer supported and is a must for me. (Please don't suggest alternatives, it has to be the final Firefox before Quantum!)
I can only foresee two potential issues;
1.) Default Firefox Windows registry entries that may get overwritten,
2.) Firefox user settings location; "\Users\xx\AppData\Roaming\Mozilla\Firefox\Profiles\xx.default".
PS: I assume you still have/allow downloads of older apps. IIRC the final official (non beta) Firefox before the December 2018 Quantum release was version 56.0.2. Where can I download the portable app version of that? If there was an even newer (non beta) version please link to that.
If it turns out there are serious conflict sharing issues, my solution, such as it is, will be to use a regular Firefox 56.0.2 (64-bit) and not upgrade it - ever, and switch my regular browsing to another browser. I'll do it if I have to!