I noticed in the default zen install, there was some whitespace below my footer in firefox.
Please note the screenshot I have attached. I made a quick modification to the theme by making the body background-color set to yellow and removing the zen default of padding: 10px;. I also set #page to back a background color of red.
It seems that all the floats were not being cleared for some reason. adding a class clear-block to page didn't completely solve the problem but appeared to help reduce the space.
I found that setting #page to have the property "overflow: hidden;" solved the problem. This probably needs some additional research and testing. I unfortunately do not have an IE box right right now to investigate this further.