When you updated to FIrefox 18, did you get any warnings during Firefox restart about the Norton bar not being compatible? By default, Firefox warns about incompatible Add-ons.

  You could go to "Tools>Add-ons" to see if it is there and / or disabled. 
  Assuming that the toolbar is actually there, compatible, but not displayed, you would go to "View>Toolbars" and select it.

I should have checked first. Actually Symantec does have an answer for you. Go to:

http://community.norton.com/t5/Norton-Toolbar-Norton-Identity/Firefox-18-Support-for-Norton-Toolbar/td-p/886714

Apparently they have a patch or upgrade there.