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)
For bugs in the Hardware Abstraction Layer (HAL) code regardless of the backend.
Example of appropriate bugs: implementation of a new feature for a backend, fixing a bug for a specific backend or general bugs regarding HAL.