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 with the spelling checker used by Mozilla-based applications. This includes the backend code, the Hunspell spell checker, the en-US Hunspell dictionary, and the osxspell interface to the Mac OS X spell checker.