My problem is similar to this, but I need a way to get the coordinates of the right side of the selection with Javascript in Firefox. I made a small example to show what I mean:

The code I got from the other post is the following:
var range = window.getSelection().getRangeAt(0);
var dummy = document.createElement("span");
range.insertNode(dummy);
var box = document.getBoxObjectFor(dummy);
var x = box.x, y = box.y;
dummy.parentNode.removeChild(dummy);
This gives me the coordinates of the beginning of the selection. Is there any way to retrieve the coordinates of the end of the selection?
Yes. That bit’s quite simple: you just need to call
collapse(false)on the Range obtained from the selection. Be aware thatdocument.getBoxObjectFor()has now been removed from Mozilla, so you need the dummy element’sgetBoundingClientRect()method instead: