Web design and Javascript Question

Hello all,

I have a small hurdle, that I'd like to solve and maybe you guys have an idea. I have a web form that when submitted shows a box (CSS and onSubmit) that the form is being processed (like to keep people from pressing the button over and over again). If a missing field is encountered the next page displays a message to correct the problem and a link back. This link back is done in javascript to give the user the back button functionality, so they don't have to type in everything again. When the user return to the page I have set an onload to hide the CSS message box again.
And here is comes: It only works in IE! Not in Safari or Firefox. Why?

I'd just to hide the box again so it can reappear when the user presses the submit button the second time.

So try it out and don't fill in all the required information.
Here is a link: www.neolithmedia.com/client/dcc/contactron
-> Click on Client Data Sheet

user: guest
pass: 2lustigen3

If you guys have any other ideas, I'm all ear.

Thanks,
Sebastian