firstAttempt.html

<script>
	if(document.location.href.match(/debug/)){
		document.write("<script src=\"external.js\"><\/script>");
	}
</script>

This doesn't work -- the document is loaded asynchronously in Firefox.

gistfile1.html

<script type="text/javascript">
	// here's our runtime variable -- checking for "debug" in querystring
	if(!document.location.href.match(/debug/)){
		document.write("<!--");
	}
	console.log("before script tag")
</script>
<script type="text/javascript" src="/external.js"></script>
<script>
	console.log("after script tag");
</script>
<script>
	/* --> <script> /**/
</script>

HTMLComments1.html

<!--
<script src="/external.js"></script>
<!-- -->

Even this doesn't quite work -- it works on Chrome, but Firefox prints out "<!--".