Thinking about it 3 plugins that should get integrated into Firefox are noscript, cookie monster and ad block plus.
noscript: It allows you to individually select sites of scripts you want to run and don't want to run. It wasn't too hard for me to explain how to use it to my mom and to others that don't really understand computers. If the page seems to contain everything you where looking for stop allowing new scripts. It can cut malicious scripts down quite a bit.
cookie monster: like noscript for cookies.
add block plus: It isn't just to block advertisement it can be used to customize your experience in a way you don't see stuff you don't want to see very nice plugin.
IMHO these should come built in to Firefox. These three plugins should really be built into all web browsers. Chrome and IE need to think about adding such features. Script selection, cookie selection and what you see selection. And allow selecting of group effort sites that have lists of stuff to block on default.
I don't mind seeing advertising, however if it contains vicious scripts I want them blocked. If it contains questionable material I want it blocked. I am happily married I don't want to see advertisement for dating sites. If I can customize what I see on a regular basis I'm happy.