I use NIS, I have a Norton Toolbar but do not use it. My tooolbar is version 5.6, that does not work with Firefox 5 or 6 . AFAIK it is not possible in many cases to check whether an extension will work until you do upgrade. Once you have upgraded there is an extension to test and report whether an extension works. But be aware Firefox policy is that you do not downgrade to firefox4 again, there are hurdles to overcome if you try.

It is possible to install firefox 5 in addition to firefox4, but again not something for the average user to try, it will mean a second custom instal and a second profile see for instance http://kb.mozillazine.org/Using_multiple_profiles_-_Firefox

Norton does seem to operate mainly using IE by preference.

Each time I install firefox updates I lose my norton also. I had norton reinstall it for me and once I signed off it was gone again. I love firefox but this needs to be addressed since I love my norton more

I use Norton 360 and it's compatible with the latest Firefox 8 version. However, you will have to activate or enable the add-ons since Firefox 8 automatically disables the add-ons for security reasons.

You are right though, some companies do take some time until they have updated their add-ons to be compatible with the latest versions of Firefox.

Norton had updates available for their Firefox add-ons on the same day that Firefox 6, 7, and 8 were released; with Firefox 4 and 5 Norton was very slow, like they were caught by surprise.