window.getSelection().getRangeAt(0).setEnd will modify the selection in FF I don't think we want to support this. But I'm pointing out here that we differ from FF in this behavior. I will file a bug with FF once I come up with a nice test case (which I will attach here). FF's getRangeAt function returns the same Range objects as were passed into addRange, meaning === returns true when compared with the range object passed into addRange. Moreover, modifying the Range object returned by getRangeAt will modify the selection. I don't know how many sites depend on this behavior (if any). My vote is that we do not support this behavior and that we convince FF to change away from this confusing API, but I'd be very interested in hearing others opinions on the matter.