Bubble Town disappears

Hi, I like to play a game on my iMac called Bubble Town on Games | Free Online Games | Free Games | Games.com. Because it is a progressive game (i.e., you try to reach higher levels of skill) I like to minimize it and leave it open while I'm not playing so I don't have to start at the beginning each time. Unfortunately, I frequently return to my iMac to find that the game has disappeared (i.e., the minimized window is no longer on the dock). Does anyone know why this happens, and what can be done to prevent it? THANKS!

Since it's a a web driven game, the game no doubt has a timeout whereby after some time of inactivity, the game most likely closes..