Icons disapearing from the dock

My parents have a MB and they keep getting both the Mozilla Firefox and Thunderbird icons disappearing from the dock when they close the application.

I have opened the apps and right clicked on 'keep in dock' but they still get removed from the dock about once or twice a week.

The only thing i can think of is that its happening because they are running leopard rather than snow leopard because it doesn't happen at all on my MBP running snow leopard.

Any suggestions

Thanks

Tom

One of them is more than likely accidentally dragging them off the dock. Which one only uses those two apps?

Has nothing to do with a different OS version.
Ask them if they ever notice the "poof" image.
It goes by quickly, so maybe undetected...