The longtime basics basics is to use the full offline setup (and not the small online stub installer) to custom install the new Firefox in its own separate folder and have that version use its own Profile and not share Profiles. A easy way is to have shortcut start with specific profile.

https://www.mozilla.org/firefox/all/

https://support.mozilla.org/en-US/kb/profile-manager-create-remove-switch-firefox-profiles

I'm looking for an entirely separate install, not just another profile to the same version.

Will your first method (be able to) do that?

If so, are there any configuration questions I should look for to enable it?

Thanks