The Mozilla Toolkit is a set of APIs, built on top of Gecko, which provide advanced services to XUL applications. These services include Profile Management, Chrome Registration, Browsing History, Extension and Theme Management, Application Update Service, and Safe Mode. (More info)
We don't habitually store icons for chrome and friends. I've seen issues with trying to use the moz-anno:favicon protocol with those as nested URIs. I'd have expected them to just 'pass through' to the actual image, at least for chrome/resource.