“Out of the box” you should be able to run more than one copy of Firestorm without doing anything. If you try and it doesn't work, then go to Preferences → Advanced and make sure that “Allow Multiple Viewers” is enabled. If it wasn't, enable it and relog.
Mac does not really like to open two instances of Firestorm at once - you will notice it wants to bring the already-open one to the front. The standard way of duplicating the Application is fairly resource-heavy, so here are two alternatives. Use Method 1 if you want to create a new instance right away. Method 2 requires some advance preparation, but once you have it set up, you'll be able to access Firestorm with any of your accounts quickly and easily.
Ctrl-click on Firestorm Viewer in your Applications folder and select “Show Package Contents.” In the folder that opens, go into “Contents” and then “MacOS.” In the MacOS folder, you'll find another Firestorm with a black box icon. Double-click this to open your new instance of the viewer. Terminal will open as well; you can let that run in the background.
With this method, you will spend some time setting yourself up, but once the work is done, logging into the viewer will never have been easier.
do shell script "\"/Applications/Firestorm-Release.app/Contents/MacOS/Firestorm\" -multiple -login yourName yourPass" quit
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