Consider:

<div id='d'></div>
<script>
document.getElementById('d').innerHTML = 'hello world';
</script>

Normally d will just changes to 'hello world'. Now, I want to write a plugin (or modify Firefox/SpiderMonkey code, whichever that's necessary) to intercept the DOM call to `d` to allow/deny the access by my own rules (In the event that it's denied, the getElementById will behave as if element 'd' doesn't exist).

I have downloaded and built Firefox. Now I can't seem to find out which part (file/folder/function) of the source code do I have to look into? Do I also need to download and build SpiderMonkey for this purpose?

Thank you so much!

Cheers,
Huy