This crash started on El Capitan and has continued on Sierra.
There is no real fix for this crash (the fix will need to come from Apple); however, switching over to using the Nvidia web drivers may stop the crashes. There is a chance of the viewer running with poorer performance.
This problem affects other games, too; for example, see constant crashes - OS X 10.11 El Capitan and OSX El Capitan 10.11 Beta (15A21). The same workaround appears to fix the problem in other games also; that is, to switch over to using the Nvidia web drivers instead of the Nvidia driver that Apple provides.
Please note that this is not a viewer-specific crash. Refer to this Firestorm Jira: [BUG-10302] [Mac El Capitan] Firestorm crash/incompatibility with Mac OS 10.11 and this SL Jira: BUG-10302 Frequent Crashes in Mac OS X 10.11 El Capitan.
Sierra comes with a new feature that allows compatible applications to create multiple tabs in one window, much like a web browser. Unfortunately, Firestorm is not a compatible application, so if the key combo (Cmd-Shift-T) is hit while you're logged in on Firestorm, whether intentionally or by accident, a second title bar (the bar that shows the viewer name, version number, and name of the account currently logged in) will appear below the regular one, with no apparent way to get rid of it.
To rid yourself of this extra title bar:
The extra bar must be red and the app title not showing when you relog. If the second title bar is showing, it will still be there when you relaunch the viewer.