An API (Application Programming Interface) is a set of features and rules contained within a software program (the application) that allows interaction with it via software rather than a human user interface.

• Firefox uses callbacks to implement these APIs under the chrome namespace. For the APIs detailed here, this allows code designed for Chrome to run virtually unchanged in Firefox.
• Extensions for Firefox are built utilizing the Web Extensions API cross-browser technology.
• To a considerable extent, Firefox's extension technology is compatible with the extension API used by Chromium-based browsers (such as Google Chrome, Microsoft Edge, Opera, Vivaldi)

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