Firefox allows JavaScript code to install search engine plugins, and supports two search engine plugin formats: OpenSearch, and Sherlock.
A PromiseWorker is a ChromeWorker except instead of calling postMessage() to send a message, you call post(), which returns a Promise. JavaScript files imported into the worker scope and main thread scope which allows posting to the worker and receiving in the form of a promise. Like ChromeWorker objects, PromiseWorker is mostly used for js-ctypes but it is not limited to that. There is no XPCOM access, similar to ChromeWorker. This answers the question "When should I use a PromiseWorker?", and the answer is, whenever you would normally use a ChromeWorker, but want postMessage to return promises.
The Plugin interface provides information about a browser plugin.