Logging out of the game using the proper method is the best way to leave the game. Please follow these instructions instead of closing your browser's window.
The following steps will help you to do it:
1. While in game, select the arrow on the right of the toolbar to access the secondary menu.
Then select the gear icon that appears on the second page to access the settings menu.
2. Select Logout from the menu that appears.
That's it! You will now be returned to the game's login page.