Greasemonkey

Greasemonkey is a userscript manager for Mozilla Firefox that enables users to customize the way web pages look and behave by running small JavaScript programs known as user scripts. When a page matching a script's metadata ranges loads, Greasemonkey injects the script into the page, allowing it to modify the DOM, alter styles, or automate tasks. Scripts are written in JavaScript and describe their target pages using a metadata block that can specify include and exclude rules, run-at timing, and permission grants.

Greasemonkey originated as an extension for Firefox and became one of the most widely used tools for client-side customization. In response to browser changes and competing tools, the project moved toward a WebExtensions-compatible architecture in later major versions, notably Greasemonkey 4, which introduced a new API surface and changed how scripts access privileged features. The extension provides a set of GM_* API functions, such as GM_getValue/GM_setValue for storage, GM_xmlhttpRequest for cross-origin requests, and GM_addStyle for injecting CSS, as well as a system for loading, enabling, disabling, and editing scripts through a user interface.

Scripts are typically sourced from community repositories and curated by users, with popular hosts including Greasy Fork and OpenUserJS after the shutdown of the older userscripts.org. While Tampermonkey and Violentmonkey offer similar functionality across multiple browsers, Greasemonkey remains a Firefox-focused option with ongoing development and support within the Mozilla ecosystem.