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)
Jeff has a rough patch that speeds up this operation substantially. Rough profile show a decrease from 5% to ~1% over a small number of samples on cnn.com panning. It needs to be cleaned up before landing.