When I set AllowMultipleInstances=True, Firefox will not start another copy. It only allow multiple instances when it is set to False. It has been like this for some time.
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That's not what it's for. AllowMultipleInstances is designed to let two different packages of Firefox run with different profiles at the same time. When enabled, you lose the ability to run FirefoxPortable.exe a 2nd time for the same instance (say to start a file). This is due to Firefox itself. There is no workaround.
Please make sure you are using a current FFP launcher (and not just an auto-upgraded copy of FF internally), as the method to do multiple instances changed and if you're using an old one, it won't work.
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