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's stuff like -moz-script-min-size, -moz-script-min-size-multiplier, and a bunch of others.
Important to note that like font-size, some of these properties use the inherited font size when calculating em/ex/ch units.