Firefox add-ons

Blocking

1. Block sites by regex
2. uBlock. Blocks more than just ads.
3. uBlock Origin. Blocks more than just ads. Supports many third party lists, and you can make your own, block a page element, and more. Smaller and more efficient than other blockers.
4. Blocksi blocks websites

Privacy

1. Ghostery. Tells you how many third-parties are tracking you.
2. Light Beam. Shows you a graphical representation of how the websites you visit interact with third parties, and track your web movements. Formerly called Collusion.
3. AnonymoX. Change your IP to appear to be from another country, show your public IP, delete/show cookies, and more.
4. NoScript. Stop all scripts from executing on your browser, and only let trusted ones through. Helps prevent several browser attacks.
5. DuckDuckGo is a search engine that doesn't track you.
6. Disconnect. Disables 2000+ trackers.

Screen capture

1. Awesome screen shot. Lets you do basic edits on a screen before copying to clipboard, or saving to a file. Edits: Boxes, circles, lines, text, in different colors.

Scripting

1. Greasemonkey. Now being developed more.

Misc

1. Down Them All