I don't think sending panel itself is a good idea cause that would allow other evil add-ons to temper with this one. If we can avoid passing panel than we should do it. If not we should use unique id's for panels that can be used to identify them.

I'm not absolutely sure, but I think [outerWindowID](https://developer.mozilla.org/en-US/docs/XPCOM_Interface_Reference/nsIDOMWindowUtils?redirectlocale=en-US&redirectslug=nsIDOMWindowUtils#Attributes) of the panels `contentWindow`
will be unique and consistent (even if documents in the panel are reloaded).

So we could either use that as identifier or generate one ourself, if you end up on generating ID's yourself make sure to make it's unique across add-ons either by prefixing or by using uuid module.