Welcome to Greasy Fork, a site for user scripts.
User scripts put you in control of your browsing experience. Once installed, they automatically make the sites you visit better by adding features, making them easier to use, or taking out the annoying bits. The user scripts on Greasy Fork were written by other users and posted to share with the world. They're free to install and easy to use.
To use user scripts you need to first install a user script manager. For Firefox, the best choice is Greasemonkey. For Chrome: Tampermonkey. Other options exist. Once you've installed a user script manager, find a script you like on Greasy Fork and click the green Install button. The script is then installed and will run you visit a site it applies to.