Portable AND regular Firefox running at the same time?

Check out the readme in PortableFirefox\Other\PortableFirefoxSource. It's Firefox itself that checks this so multiple versions aren't running into each other. There's a hack you can use to set an environment variable to keep it from doing this. The launcher uses this hack when you set the INI file to do so.

This isn't necessarily recommended in normal use and may cause issues within Firefox itself as it isn't a normally-used featured and isn't heavily tested.