not sure if this is the _best_ way to solve the problem, but I added a c-string version of |NS_NewStringInputStream|, which can't use overloading since it's |extern "C"|, so I named it |NS_NewCStringInputStream|.

xpcom/io/nsIStringStream.cpp

@@ -265,6 +265,12 @@ class ConstStringImpl
                                         {
                                         }
 
+                                        ConstStringImpl(const nsCString& inString)
+                                            : ConstCharImpl(inString.ToNewCString(),
+                                                            inString.Length())
+                                        {
+                                        }
+
                                         ~ConstStringImpl()
                                         {
                                             Recycle((char*)mConstString);
@@ -381,6 +387,27 @@ extern "C" NS_COM nsresult NS_NewStringInputStream(
     return NS_OK;
 }
 
+//----------------------------------------------------------------------------------------
+extern "C" NS_COM nsresult NS_NewCStringInputStream(
+    nsISupports** aStreamResult,
+    const nsCString& aStringToRead)
+    // Factory method to get an nsInputStream from a cstring.  Result will implement all the
+    // file stream interfaces in nsIFileStream.h
+//----------------------------------------------------------------------------------------
+{
+    NS_PRECONDITION(aStreamResult != nsnull, "null ptr");
+    if (! aStreamResult)
+        return NS_ERROR_NULL_POINTER;
+
+    ConstStringImpl* stream = new ConstStringImpl(aStringToRead);
+    if (! stream)
+        return NS_ERROR_OUT_OF_MEMORY;
+
+    NS_ADDREF(stream);
+    *aStreamResult = (nsISupports*)(void*)stream;
+    return NS_OK;
+}
+
 //----------------------------------------------------------------------------------------
 extern "C" NS_COM nsresult NS_NewStringOutputStream(
     nsISupports** aStreamResult,

xpcom/io/nsIStringStream.h

@@ -33,6 +33,13 @@ extern "C" NS_COM nsresult NS_NewStringInputStream(
     // Factory method to get an nsInputStream from a string.  Result will implement all the
     // file stream interfaces in nsIFileStream.h
 
+//----------------------------------------------------------------------------------------
+extern "C" NS_COM nsresult NS_NewCStringInputStream(
+    nsISupports** aStreamResult,
+    const nsCString& aStringToRead);
+    // Factory method to get an nsInputStream from a cstring.  Result will implement all the
+    // file stream interfaces in nsIFileStream.h
+
 //----------------------------------------------------------------------------------------
 extern "C" NS_COM nsresult NS_NewStringOutputStream(
     nsISupports** aStreamResult,