Immediately after the upgrade to NIS2010 ver 17.1.0.14, the following happened in Firefox:
1. Add-on's no longer
a. appeared on screen
b. appeared on View>Toolbars
c. could be enabled in the add-on section
d. downloaded and installed
This was true after numerous reboots.
As an experiment, I disabled both Norton Add-On's. At that point:
a. all other add-on's started working EXCEPT for the two Norton Add-on's, neither of which could I enable.
b. I was unable to uninstall either of the two Norton Add-on's even after disabling NIS' Browser Protection, Safe Surfing, and Identity Safe.
2. Firefox 3.5.4's Full Screen option was broken. Pressing F11 took me to a full screen, but when I pressed F11 again, the new screen lacked the blue bar at the top. Various configuration's of the original screen options (full/user shaped) had no impact on the problem.
Both problems persisting even after I uninstalled and reinstalled Firefox, maintaining the database for the original settings. I suppose I will have to try a full uninstallation of Firefox, which I do reluctantly because of the time problem of rebuilding it the way I want (in particular, going away the add-on's I like). This will have to wait until tomorrow. I want to see what other users experience.
Edited Section:
I just discovered the lengthy thread on problem 1. above. I will still leave it in as a simple summary of what is in the other thread for those who want to get there quickly. 
However, no one has mentioned problem 2. above. I hope someone will try the F11 option in Firefox and see what happens for them. Meanwhile I am reporting this on the Mozilla bug site.