Honestly, Norton's tools are junk, you don't gain any additional security by using them and they can cause alot of problems, you are better off just uninstalling them.

If you do want to keep using norton, https://community.norton.com/t5/Product-Update-Announcements/Firefox-26-Support-for-Norton-Toolbar/ba-p/1062143 might help

I just want security for my Mac pro. When you say additional security are you saying I already have it?

I'm saying that the security in Norton's toolbar (the part complaining that Firefox and it aren't compatible) won't provide any security that Firefox doesn't already have. Norton's actual anti-virus program runs independent of the browser you use and so isn't part of this. You should remove the norton Firefox extensions, and leave the norton anti-virus component

Norton confidential is a old product that Norton killed off, which explains the out dated messages about Firefox 1.5. It has been replaced by Norton Identity Safe. https://identitysafe.norton.com/