This question gets asked pretty often, so I'd been meaning to write something up for a while. Browser extensions were one of the first ways I got interested in free software, and there's a lot of really useful ones out there.

If you want even more options, here's some I have installed but disabled.

• Forcastfox (fix version) - Shows the weather.
• Gesturify - Control the browser with just mouse gestures. I don't really use it because I try to use keyboard shortcuts, but it's really handy if you use a mouse a lot.
• GNU LibreJS - Blocks any non-free javascript. This is not a easy or fun extension to use. I've got my one system set up to use only free software, with this enabled. It really shows you how much proprietary javascript you're using.
• Picket Line Notifier - It tells you what products are from companies with ongoing strikes.
• Kiwix - Download offline sections of wikipedia, stackoverflow, khan academy and a bunch of other websites. They also have desktop and phone apps, so I don't really use the browser extension.
• Buster: Captcha Solving for Humans - I love how there's an add-on bot that can solve captchas to prove you're a human.
• Allow Right Click - Turn this on when a website tries to block you from saving images.
• User Agent Switcher and Manager - Occasionally you find a site that doesn't work in your browser, but just changing the user agent string often fixes it.