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)
It's been observed that JSContext::markAtom and related methods are easily confused with GC marking functions (which are related but not the same).
A more informative but more verbose name would be 'markAtomAsUsedByZone'.