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)
Caps is the capabilities-based security system for the browser. Manages principals (entities responsible for a piece of code) and the protected capabilities (the ability to perform a restricted action) that may be enabled for given principals.