Shouldn't really be working in FF.
When you call a <form>s submit() function, you *BYPASS* the onsubmit!
I suppose it is working in FF because you have TWO bugs. You are *BOTH* calling the <form>'s onsubmit() function *AND* you are *NOT* suppressing the action of the submit button. So FF ends up submitting the form *TWICE*.
It's an easy fix:
<input type="submit" name="okButton" id="okButton" class="shortButton" value="OK"
if you simply do return true
from a submit button, then the <form> will be submitted as normal and the onsubmit function will be called.
Notice also how you can shorten your code that disables the form fields.