pculture/miro — comment on commit 311596857e249ffcf57ca4bf06d2b01cf83605b2
File: tv/windows/plat/frontends/widgets/XULRunnerBrowser/MiroWindowCreator.cpp line 119
By bendk:

Also, at the end of all of this, you may need to call NS_IF_ADDREF(_retval).

I'm never sure about reference counting an XPCOM, but at the end of the function, the caller expects that you have addrefed the return value exactly once.