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