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 Mozilla's modular networking library (aka "Netlib" or "Necko".) The networking library supplies the software interface that Mozilla uses to access physical transports (e.g. the Internet and local drives), perform URL resolutions, and handle a variety of networking protocols.
Examples of appropriate bugs: URLs with backslash not fetched; URLs starting with a single slash turn into http:///; Cannot access authenticated FTP site.