Replace JS and refresh page?

[R. Rajesh Jeba Anbiah]

I'm just trying to do a Firefox extension for something I want to
achieve: The case is something like the following... I want to replace
a random script block with other _but_ before the page is get loaded.
I'm lost in this place: how to "refresh" the whole page after
replacing the script block, _but_ before anything get executed. Any
suggestions are welcome. TIA.

<!---script block 1--->
<script>
document.write("Hello-1");
</script>

<p>foooooooooooooooooo</p>

<!---script block 2--->
<script>
document.write("Hello-2");
</script>