The list of threads always sorts so that threads with new posts show up first. However when viewing posts within a thread its always in conversation order, that is the order they were posted. Currently there is no feature to customize this sort and it really doesn't make sense for the sort to be in reverse order as that makes it strange to read the conversation. It makes more sense to read the posts in the order they were posted in order to understand the conversation.

When you reply to a thread, the edit page does show the posts in reverse order (below the editor) because presumably you are responding to the most recent post in the thread.

Joe