fix for bug #71879, r=dveditz, sr=alecf

modules/libpref/src/nsPref.cpp

@@ -635,7 +635,8 @@ static int PR_CALLBACK
 NotifyObserver(const char *newpref, void *data)
 {
     nsCOMPtr<nsIObserver> observer = NS_STATIC_CAST(nsIObserver *, data);
-    observer->Observe(observer, NS_LITERAL_STRING("nsPref:changed").get(),
+    observer->Observe(NS_STATIC_CAST(nsIPref *, nsPref::GetInstance()),
+                      NS_LITERAL_STRING("nsPref:changed").get(),
                       NS_ConvertASCIItoUCS2(newpref).get());
     return 0;
 }