When you try to start two different Firefox versions at the same time (like stable and nightly) you'll get a message that you can only run one instance of it at a time.

Here's how to get around that:

  1. First open the first Firefox version as usual
  2. Now start the second one using the Terminal like this: open /Applications/FirefoxNightly.app --args -ProfileManager (On *nix systems other than macOS it's enough to just run the executable with -ProfileManager)
  3. Create a new profile in the now open window and either start using it right away or open it with -no-remote -P [profilename]

Now you should be running two versions of Firefox simultaneously.

More info on Firefox arguments can be found here and the blog post where I got this from here.