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 related to media (video/audio) -- especially when the reporter is not sure which area of the media stack the problem is in. This category will mostly be for untriaged video/audio issues. Bugs in this category will typically be moved to another media category during the triage process.