You can run multiple instances of the application using the terminal, but you need to run the application binary located in the Contents/MacOS/ folder
Launch a terminal and enter the following command:
/Applications/Safari.app/Contents/MacOS/Safari &
It will launch a new instance of Safari.
But for Firefox, this trick will not work, because Firefox will complain that it can work only one instance at a time. Other applications, such as iTunes, themselves will control this to limit you to only one instance running at a time.
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