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 problems with HTML Form Elements: buttons, images, edit fields, passwords, file controls, submit buttons, selects, and textareas. NOT for file upload dialog problems, those belong in XP Toolkit/Widgets.