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.

This can be annoying if you quickly want to test a certain bug in you application with multiple versions or try to see if a certain behavior is present in another version.

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
    • You might need to change the path to Firefox in case you don’t want to use the nightly.
    • (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 start Firefox again with --args -no-remote -P [profilename]

Now you should be running two versions of Firefox simultaneously.

More info on the Firefox command line arguments can be found here, the blog post I got this from here.