The Best Browser Extensions for Security and Privacy

A thorough article from Lifehacker - The Best Browser Extensions that Protect Your Privacy:
But note that some of these tools need to be used carefully as they can cause some browser hiccups.
In summary:

Block Ads, Scripts, and Popups with AdBlock Plus

Stop Everyone from Tracking You with Disconnect

In addition to privacy protecting tools and ad blockers, Lifehacker documents some other add-ons, utilities, and services worth considering if you have the other bases covered including:

HTTPS Everywhere (for Firefox/Chrome) which will try to find secure versions of the sites you visit.

And Hotspot Shield, Hideman, or Tunnelbear. for a Virtual Private Network or VPN. This encrypts all of your internet traffic.