14.04 freezes after upgrade

[Linda]

Any suggestions of what logs to look in to figure out why 
14.04 LTS is freezing. I did an upgrade from 12 and 
everything seemed to be working well, but if I try and open 
Firefox the machine freezes you can't get a terminal or use 
ctrl alt bksp , which is set to restart X and have to turn 
off the machine and reboot. I removed firefox and 
./mozarilla in the home directory and reinstalled. It then 
opened firefox but froze again shortly there after.  I could 
just start over and do a clean install, but I'm hoping to 
make the upgrade work. It is running gnome fallback (metacity)
                  Thanks
                   Linda