Shared components used by Firefox and other Mozilla software, including handling of Web content; Gecko, HTML, CSS, layout, DOM, scripts, images, networking, etc. Issues with web page layout probably go here, while Firefox user interface issues belong in the Firefox product. (More info)
The embedding API is the set of functions that an external application uses to host an instance of the layout engine within itself. The embedded layout engine provides services for rendering web content (HTML, XML, CSS, etc).