This tag is to be used for questions about add-ons, specifically about using them with the Tor browser and about why it is unsafe.

Add-on is the term for software modules that can be added to web browsers and related applications. There are three types: extensions, themes, and plug-ins. Add-ons allow developers to extend and modify the functionality of the user's browser. They are written using JavaScript, HTML, CSS and some additional JavaScript APIs. Add-ons can change the appearance or content of particular websites, modify the browser's user interface, or add new features. Although add-ons may seem harmless, some add-ons track their users and collect data on them (to sell to others), insert ads into websites, drive traffic to certain websites, or even install malware/spyware on their devices. For these reasons, the Tor browser strongly advises against using addons with the Tor browser.