Browser extensions allow additional features to be added to browsers through code added via extension APIs, as extensions have evolved from plugins and addons used in early browsers like Firefox and now provide powerful yet easy-to-use ways to customize Chrome, Firefox, and other modern browsers while leveraging web technologies like HTML, CSS, and JavaScript. Extensions are commonly used to integrate services, implement new specs, and add customized features to browsers, though developers must take care not to overuse extensions in ways that abuse their capabilities.