Bug 820182 is making nsISupportsArray.ElementAt go away. I found two instances of it in mailnews, so lets kill those whilst we're not in a rush. One is in the rdf datasources, and we can just adapt for it (replacing nsISupportsArray isn't practical there yet), and the other one is also a reasonable size replacement, so in that case, we'll just make it easier for when we get to that bug.
Created attachment 691913 [details] [diff] [review] The fix
Comment on attachment 691913 [details] [diff] [review] The fix (I'm not even sure whether we're still using the RDF command mechanism.)