Related to Bug #939318 When switching to offline mode the DNSserver is set to offline mode, and when subsequently going online again DNSService->Init() is called. The latter however basically have no effect at that point. This will lead to the old cache surviving. When for example using managed offline mode and switching to offline and online automatically when networks change, that's bad. Possibly, a short-cut is to do mDNSService->Shutdown(); from nsIOService when going offline as that will force a proper re-init to happen on the subsequent online action.
This is fixed now with bug 939318 being resolved.