Unable to start firefox profile manager on MacOS
To start the Profile Mnager, Firefox must not be running.  Also, the command to start Profile Manager from the command line is:

/Applications/Firefox.app/Contents/MacOS/firefox -ProfileManager


Note, that it's only one dash preceding the option and you don't need to use firefox-bin.  You can also use the short hand version, firefox -P as well.  

Mozilla has excellent documentation on its Command Line Options.