That's correct. Marshal.Release works on an IntPtr to the IUnknown interface of the COM object which you obtain with Marshal.GetIUnknownForObject.
I'm not very good at COM Interop as I've only used it a few times (and that is few years ago now) and I know nothing of the possible differences between COM and XPCOM so I'm out of ideas now. My last desperate suggestion is to try a call to GC.Collect after Marshal. ReleaseComObject or Marshal.FinalReleaseComObject