nsIHandlerInfo has support for checking for a default handler and getting the default handler's name, but you cannot get the icon of the default handler. This should be an easy fix once we decide the interface changes required. nsIHandlerInfo / nsIMimeInfo *do* internally get the actual path to the default handler, so we could easily make a "defaultHandlerIcon" property that holds a "moz-icon://" URI to the application. The user of the property could append the "?size=16" or "?size=32" depending on the given need.