Tampermonkey

Tampermonkey is the premier userscript manager (browser extension) for running custom JavaScript programs to modify and enhance any website's functionality.

Tampermonkey is a high-performance browser extension, available for Chrome, Firefox, Edge, and Safari, with a user base exceeding 10 million. This tool allows you to inject userscripts (small JavaScript programs) into web pages, fundamentally altering their behavior or appearance. You gain control over the DOM: use it to remove unwanted ads, implement custom CSS styling, add new keyboard shortcuts for efficiency, or automatically populate complex form data. The extension provides a robust dashboard for managing scripts, features automatic updates for security, and supports synchronization across devices via cloud services (Google Drive, Dropbox). It is the essential utility for deep, client-side web customization.

https://www.tampermonkey.net