Firefox button disappears from taskbar

[dah]

Sometimes when I open IE after opening firefox or when I open multiple instances of Firefox, some instances disappear from the tasbar. I can use alt+tab to find all instances, and if I pick one of the instances that have disappeared from the taskbar, it will then appear on the taskbar. I do not group similar taskbar buttons. This behaviour also occurs when I open a Java applet. Any thoughts or has anyone experienced similar behaviour. I have search the forum and did not find any mention of this behaviour. Thanks.