Add-on is the Mozilla term for software modules that can be added to the Firefox web browser and related applications. Mozilla hosts them on its official add-on website. Browser extensions are the primary type of add-on. In 2017, Mozilla enacted major changes to the application programming interface (API) for extensions in Firefox, replacing the long-standing XUL and XPCOM APIs with the WebExtensions API that is modeled after Google Chrome’s API. Thus Firefox extensions are now largely compatible with their Chrome counterparts.