Enclose Selection With Html?

I am looking for a working replacement
of the following function for Mozilla (this was written for IE):

// encloses the selected area into starting and closing tag
// if no closing tag is given the starting tag will replace the selection
function encloseSelection(startingTag, closingTag)
{
editRange = editArea.document.selection.createRange();

if (closingTag) {
editRange.pasteHTML(startingTag+editRange.htmlText +closingTag);
return;
}

//editRange.collapse(false); // move insertion point to end of text range (append tag)
editRange.pasteHTML(startingTag); // replace selection with startingTag
}

Longer version:
I am working on making an older "WYSIWYMG"-editor (written for IE)
Mozilla-compatible. Everything works fine so far, but I am not able
to figure out a solution to the above problem.

I'd like to enclose a selected text in one of the famous
"designMode=on" iframes with HTML-strings. I've tried numerous
ways - by now without luck.