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