Last time, there is a Firefox addon, that is MinimizeToTray. Since I am using portable Firefox, it requires some time to start. Using MinimizeToTray, I just minimize it without exit, so that I can open the Firefox later without starting the Firefox again. However, Firefox 3 does not support MinimizeToTray. This is sad.
But, I just found a powerful freeware, that is Tray It!. Using it, you can just minize any application to the system tray. I like it. Because I am using portable OpenOffice also. Like Firefox, it requires some time to start. Using Tray It!, both Firefox and OpenOffice can minimize to tray without using the space of taskbar.
But, beware that, using Tray It!, press the “x” button does not close the application, you need to go to File>Exit, only that the application will exit.