Firefox usually says to check with your security software program provider - to find out about compatibility issues.

I found this about FF5 + Norton - hope it helps - I am staying with the older version of FF for now.
http://us.norton.com/support/kb/web_...635EN&ln=en_US
and this-
http://community.norton.com/t5/Norto...ix/td-p/515574


I'm still running FF 3.17 due to my security program (Zone Alarm Extreme)- I lose the browser force field protection if I update to the newer FF.