I need a script to generate a popup when page loads with background disabled. Please help

I've used following code and it works fine with IE but not with fire fox.

function editmobile()
{
document.getElementById("mobile").readOnly=false;...

I have a form on which there is a textbox for entering mobile number. Below this I have a check box saying 'Verify number' on selecting which user gets sms for validation code. Now on the update...