About Tampermonkey
Tampermonkey is the userscript manager most people start with, and the one most scripts are written and tested against. The Chrome listing shows about 12 million users, and it also runs on Edge, Firefox and, as a paid app, Safari. It installs, updates and switches the small JavaScript programs that run on sites you name.
It has a built-in code editor, per-script settings and a broad set of GM_ functions that scripts use to store data, make requests and add menu items. Scripts can sync between browsers and update automatically. The Chrome version runs on Manifest V3 and uses Chrome's userScripts permission, so since version 5.3 you must switch on the Allow User Scripts toggle (Chrome 138 and later) or Developer mode for it to run anything. Its FAQ covers both.
The trade-off is openness. Tampermonkey was open source until version 2.9 in January 2013 and has been proprietary since, so you rely on the developer's word for what the extension does. Its FAQ says the author has no interest in users' data, and that it needs access to every page because it cannot know where your scripts will run. The scripts you add are the real risk. For most people, it is the easiest place to start.
Pros and cons
- Available for Chrome, Edge, Firefox and Safari
- Manifest V3 build for Chrome, updated in October 2026
- Built-in editor, sync and automatic script updates
- Largest user base, so most scripts are tested with it
- Closed source since 2013, so the code cannot be read
- The Safari app is a paid download
- Needs a user-scripts setting switched on in Chromium browsers
Permissions
It requests access to all websites, because scripts can target any page, plus the userScripts permission that Chrome requires for running them. Tabs, storage, cookies and downloads access support what scripts are allowed to do through the GM_ functions.
