Some Mac users may have the potentially unwanted software, PremierOpinion, present in their system. PremierOpinion hooks into the browser and causes consistent crashing at startup after updating to Firefox version 46.
Affected users should perform a scan for malware with an external tool like the free version of Malwarebytes Anti-Malware for Mac or follow a manual removal guide to get rid of PremierOpinion, see: http://www.thesafemac.com/arg-premier-opinion.
If you need further assistance or have technical issues with a different crash, you can refer to Troubleshoot Firefox crashes (closing or quitting unexpectedly).