Norton hasn't updated their Firefox extensions yet.

A Norton 360 user has reported that the eta for an update is sometime in May. Those Norton clowns just can't see their way clear to just support the browsers that their users want to use, they always want to roll the update for Firefox into a new release of theirs and charge you for the new version. (They claim they are extending your existing agreement, but from some of the reports I have read, they don't really do that.)

If you want to revert to 3.6.16 see this.
http://support.mozilla.com/en-US/kb/Installing+a+previous+version+of+Firefox

3.6.16 is available here.
http://www.mozilla.com/en-US/firefox/all-older.html

Thank you edmeister. This has been helpful to know that we have to wait for Norton to upgrade their extensions. It is very helpful to know the website to get the old version 3.6.16

Thanks