Strangely this is broken only in Firefox and Opera (IE, Chrome and Safari works as it should).
Any suggestions for a quick fix?
I have found another "solution" to this problem.
When declaring your sortable, you should add a cancel: option like this:
One apple
Two pears
Three oranges
This version does not force your cursor at the start, but you also can't drag using this field. I'd advice wrapping the in a container which has some sort of dragging handle (like the dots at the start of every row in gmail).
side remark: ':input,button' is required, because that is the default option. If you do not add these, you get similar problems with inputfields and buttons.