This is old and obsolete attempt of the Plugin DK, it uses xpcom plugin API which is currently not recommended. The new DK will be added under modules/plugin/tools/sdk. See bug 93048.
Bug 93048 seems to be about something completely different. Is this directory ready to disappear now?
I am sorry, my bad. Bug 90348. And yes, this folder should disappear as soon as possible, so that people don't try to use examples from it.
Ok, they're gone.