jQuery's focus() in firefox

I have a form with 1 textfield, on pressing tab the value in input is saved and the form refreshes.
the first time I open the form there's a focus on the input text (I have this jQuery('#input_x').focus(); when creating the form code)
but when the form refreshes the focus is not on the field.
what's strange to me is that when I run "jQuery('#input_x').focus();" in firebug console the focus doesn't return to that input either...
btw this works with chrome

Is this code inside the $(function(){...}); or $(document).ready(function(){...}); or in script tags at the end of the html file?

<script>jQuery('#input_x').focus(); </script>
form's html....

well it's a bit more complicated.. but the fact that it works the first time the form is loaded but not for the second time, confusses me..
what's more confussing is that direct run of the jQuery('#input_x').focus(); in firebug's consule doesn't work..