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)
This is a meta bug for all of the Firefox Core Engineering work that falls under the rubric of getting more information from our audience(s).
(Sub-metas may exist for intensive efforts, like client-side stackwalking.)