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)
With -moz-box-sizing: border-box, if I get the computed height off an element with padding, then set the style.height on the element, it shrinks. I'm not sure if/how box-sizing should affect JS this way, but it sure is surprising.