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)
Cross platform widget interfaces and cross platform base implementations. Bugs that belong in this component are cross-platform problems and API issues. Bugs like this that only occur on one platform belong in the appropriate component listed below.