[CentOS] C5.11 Firefox Upgrade = Firefox will not start

Upgraded Firefox due to today's security announcement.

Closed Firefox and restarted but after about 20 seconds, Firefox stopped trying to start. No error messages displayed.

Firefox would not start in safe mode. Reluctant to try Firefox with complete reset as might loose passwords and other settings made in about:config

Have downgraded Firefox

Removed: firefox.i386 0:31.5.3-1.el5.centos firefox.x86_64 0:31.5.3-1.el5.centos

Installed: firefox.i386 0:31.5.0-1.el5.centos firefox.x86_64 0:31.5.0-1.el5.centos

Have looked in /var/logs but can not see anything relevant.

How can I identify the cause of the problem ?

Thanks,

Paul. England, EU. Je suis Charlie.