In order to get what you want to happen try this [all one line]:
start firefox www.google.com & start "C:\Program Files\MultipleIEs\IE6\IEXPLORE.exe" www.google.com & start "C:\Program Files\MultipleIEs\IE55\IEXPLORE.exe" www.google.com
The
& is used to start more than one process using the same command line. Using
&& will allow you to start additional processes
only if the previous process started without issue.