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:
- First open the first Firefox version as usual
- 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
- 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.