uBlock. A lightweight ad-blocker that is faster than the competition. If you use Ad Block or Ad Block Plus, you can see that it delays the loading of websites quite significantly. I use Ad Muncher as well, which is a host-based solution that you have to install as a separate application, not as an extension. Ghostery. Blocks social media and advertising trackers.
HTTPS Everywhere. In light of all the hacking going on, it's a good idea to enhance your security. This extension redirects you to the encrypted version of all websites.
ScriptBlock. Do you hate the stupid ads that pop up when you're viewing a website? ScriptBlock controls the scripting language that makes it all possible. It's easier to use than the other script blocking extensions. When you access a domain for the first time, you will have to allow that domain. Then you should be okay.