如何在Firefox和旧IE版本的contenteditable div中设置插入位置

I am trying to set the caret position in a contenteditable div. Here is code that works in modern IE, Chrome and Safari, but does not work in Firefox 36.0.4 or old IE versions (9 or less). For this answer it is safe to assume that the contenteditable div contains only text (i.e no child elements other than a single text node).I am trying to set the caret position in a cont