You gotta try this one plugin, it'll change your life I swear
Rafe has recently started using the NoScript plugin for Firefox and he points out how different it makes the browsing experience to have Javascript be a per-site optional thing. I've had the same experience. Surfing ad-support sites without NoScript installed can be a miserable experience. With NoScript the web is (mostly) fast, clean, and usable again. I don't really have anything to add to what Rafe already put well, but I just wanted to note how much I agree with his assessment.