This page lists APIs that only run in Gecko Chrome code (and sometimes in other privileged circumstances).

Note: Most of the APIs exposed to the Web in general are also usable in Chrome code: see Web APIs for a list of these.

Browser API
The HTML Browser API is an extension of the HTML <iframe> element that allows web apps to implement browsers or browser-like applications. It currently works in (privileged) chrome code on Firefox desktop (version 47 and above).
ChromeWorker
If you're developing privileged code, and would like to create a worker that can use js-ctypes to perform calls to native code, you can do so by using ChromeWorker instead of the standard Worker object. It works exactly like a standard Worker, except that it has access to js-ctypes via a global ctypes object available in the global scope of the worker. Examples of ChromeWorker's using js-ctypes are availabe on Github and are linked to from the See Also section below. To use a postMessage with callback version of ChromeWorker that features promises, see PromiseWorker.