I'm running Firefox on RHEL 6.7. I'm running over X11 using a putty session. Firefox runs as root but not as a regular user even though /usr/lib64 and everything under it have even been secured with chmod 755 just to be sure it's not being denied access to some directory or file. Under regular user it doesn't stop halt with any errors. I've tried safe-mode and still nothing. But as root it starts just fine. I won't be able to provide any more details until Tuesday as I don't currently have access to the system in question.