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 plaintext or HTML editing support in Gecko. This covers the editing support behind things such as <input type=text>, <textarea>, contenteditable/designMode, and XUL textboxes.
Examples of appropriate bugs: After inserting a image, selecting Undo crashes Firefox; After deleting a word, the caret disappears.