Is your game opened up in one of many tabs in your Firefox browser? Have you closed the tab with the game (or click on your user name in the top right corner and then select Exit the World at the bottom).
What exactly is happening when you think the game is starting in the background; music/sound playing?
Sorry, the problem fixed itself eventually. The game was starting up automatically in the background without opening a tab. I knew because I could hear the log in and game sounds and eventually the sound of my character going to hell. It happened every time I opened firefox. Rebooting the computer did not help.
I'm guessing it was a problem with firefox. Firefox has a habit of running into a problem and taking a while to work through it.