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)
File bugs here for all issues related to the Gecko/Firefox build config and build system. This is the component for most issues related to Makefile files, moz.build files, configure, packaging, mozboot, mach build, mach bootstrap, mozbuild, and many other related components. If you run into an issue building Firefox, Fennec, or xulrunner, file bugs here.