Currently constructors / classes exposed by an add-on sdk can not be subclassed / extended. Fixing this would enable users to subclass Panel, Widget etc... to implement MegaPanel, MegaWidget, ... custom APIs that have more functionality.
We use core/heritage pretty much everywhere now except Windows/Tabs which is bug 854982, and widgets which will be removed.
I'm not sure we can say is it fixed. Simply using `Class` doesn't makes our APIs extendable. To be more precise, we can extend `PageMod` for example, but we can't access to the content's sandboxes from the subclass, so it's pretty useless, we basically obtain the same functionality to "wrap" the `PageMod` instead of subclass it. So, if this bug is about using `core/heritage` in all our APIs I agree can be considered fixed (but then let's update the title); otherwise I don't think it should be marked as resolved.