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 will stop the crashes. There is a chance of the viewer running with poorer performance.
(This crash has continued on Sierra.)
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.
If some characters now display as boxes, you can edit the fonts.xml file within Firestorm to cause the characters to load properly.
<os name=“Mac”>, insert this line:
<os name="Mac"> <file>ヒラギノ角ゴシック W3.ttc</file> ///(only insert this line and leave everything else as is.)/// <file>ヒラギノ角ゴ Pro W3.otf</file> <file>ヒラギノ角ゴ ProN W3.otf</file> <file>AppleSDGothicNeo-Regular.otf</file> <file>AppleGothic.dfont</file> <file>AppleGothic.ttf</file> <file>华文细黑.ttf</file> </os>
NOTE: This is only necessary for Firestorm versions 4.7.3 and older.