WOT...
https://addons.mozilla.org/en-US/firefox/addon/3456
just click "add to firefox" to install an addon.
Addons can be easily altered/removed from tools>addons>extensions
There are many more addons to be found at
https://addons.mozilla.org/en-US/firefox/
I'd recommend you try one or two at a time - if you like it keep it, if you dont then uninstall it. Then try another. Some are higher rated than others - try searching for "recommended firefox addons"
I currently have over 20 which i find useful to myself and experience no real slow-down as a result.
Norton generally wont find adware as that is not it's specialisation (it is mainly a viruschecker/firewall)
Good Luck