The menu jumps when I try to animate

In This link http://tradeera.com/demo/a.html
the menu jumps when u hover over "apple" in Firefox 3.0.3.

We want a look similar to http://www.colourlovers.com/ when we put our mouse on "Forums" then the menu moves up

I can't understand the reason for the same?

Thanks in advance

CSJakharia