As suggested ensure all firefox components are gone. This is unfortunately not easy and as I have experienced myself with hidden files and folders on the HD and user profiles that are not removed by the firefox uninstaller especially if they are an heritage of previous firefox versions.
Step-wise you need to perform this while ZA is not on the system and only when a clean version of firefox is in place you can install ZA.
**ALSO ensure the ZA toolbar is removed as the ZA installer may not always remove it. Go to your windows 7 unistall a program (of XP add/remove a program), scroll down and remove the entry: "ZoneAlarm Security Toolbar".