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)
There are currently a couple of situations where we want to store some persistent data where we use prefs instead of another mechanism. One instance of these is the graphics blocklist, I'm sure there are others.