How do you manipulate selected text via a Firefox extension

selectedText = document.selection?document.selection.createRange().text;

var userSelection;
if (window.getSelection)
    userSelection = window.getSelection();
else if (document.selection)    // should come last; Opera!
    userSelection = document.selection.createRange();

var rangeObject = getRangeObject(userSelection);

function getRangeObject(selectionObject) {
    if (selectionObject.getRangeAt)
        return selectionObject.getRangeAt(0);
    else { // Safari!
        var range = document.createRange();
        range.setStart(selectionObject.anchorNode, selectionObject.anchorOffset);
        range.setEnd(selectionObject.focusNode, selectionObject.focusOffset);
        return range;
    }
}

...