nsIInstallLocation

From MDC

Interface representing a location where extensions, themes, etc. are installed.

Defined in toolkit/mozapps/extensions/public/nsIExtensionManager.idl#56.

You can get the install location of a particular add-on using nsIExtensionManager interface:

var il = Components.classes["@mozilla.org/extensions/manager;1"]
                   .getService(Components.interfaces.nsIExtensionManager)
                   .getInstallLocation("add-on id")