When launching Mumble using the "Join mumble" button (on Firefox 27.0.1) you are put directly inside your lobby number's room on mumble, ie. #41582 or #51725
When doing this while having Mumble already open or being directly connected to the lobby server will make you join at the root room of the server (Under TF2 Center Mumble Server) instead of your lobby's room or leave you where you are if you are connected to the server. The same will happen if you're connected to another server and then join mumble, you'll be on the root room instead of your lobby's room.