ON 20071021@1:08:14 PM at page:
http://techref.massmind.org/techref/language/html/ib/Scripting_Reference/trange.htm#39376.5473842593
James Newton[JMN-EFP-786] Says

In FireFox, or other Mozilla based browsers, most of that is accomplised with:
<PRE>
textarea.selectionStart = start;
textarea.selectionEnd = end;
or
textarea.setSelectionRange(start, end);
</PRE>

Here is an example that first finds where the selection (if any) is, and then inserts some text around it and finally reselects the text after the insertion. 

<PRE>
&lt;a href="javascript:void(0);" onclick="surroundText('[b]', '[/b]', document.post_form.msg_body); return false;"&gt;</PRE>
<BR>
where surroundText is defined as:
<BR>
<PRE>
function surroundText(text1, text2, textarea)
{
	// Can a text range be created?
	if (typeof(textarea.caretPos) != "undefined" && textarea.createTextRange)
	{
		var caretPos = textarea.caretPos;

		caretPos.text = caretPos.text.charAt(caretPos.text.length - 1) == ' ' ? text1 + caretPos.text + text2 + ' ' : text1 + caretPos.text + text2;
		caretPos.select();
	}
	// Mozilla text range wrap.
	else if (typeof(textarea.selectionStart) != "undefined")
	{
		var begin = textarea.value.substr(0, textarea.selectionStart);
		var selection = textarea.value.substr(textarea.selectionStart, textarea.selectionEnd - textarea.selectionStart);
		var end = textarea.value.substr(textarea.selectionEnd);
		var newCursorPos = textarea.selectionStart;
		var scrollPos = textarea.scrollTop;

		textarea.value = begin + text1 + selection + text2 + end;

		if (textarea.setSelectionRange)
		{
			if (selection.length == 0)
				textarea.setSelectionRange(newCursorPos + text1.length, newCursorPos + text1.length);
			else
				textarea.setSelectionRange(newCursorPos, newCursorPos + text1.length + selection.length + text2.length);
			textarea.focus();
		}
		textarea.scrollTop = scrollPos;
	}
	// Just put them on the end, then.
	else
	{
		textarea.value += text1 + text2;
		textarea.focus(textarea.value.length - 1);
	}
}
</PRE>