What are the best Firefox add-ons?

I was wondering what everyone was using in terms of privacy/ad blocking add-ons in FF. I didnt realize how many trackers were on my everyday sights that I thought were okay... I was using a few but I dont know if they contributed to slow demise of my system leading me to reinstall Win7 or not... I know nothing about these, thus leading me to ask you guys.

Thanks.