Spent the entire morning troubleshooting this.
KB2670838 prevents FF from starting except in safe mode.
I downgraded from 19.0 to 18.0, same problem.
Went thru my entire system, reinstalled FF, nothing. The only difference in my system this AM was the MS update above. I removed it, rebooted and FF worked fine.
You can install that update and use Firefox if you Disable Hardware Acceleration in Firefox Turning off hardware acceleration