selection.modify() does not work when "move" is specified as first argument.
var selection = window.getSelection();
selection.modify("move", "forward", "sentence");
This works in Firefox
Selection::modify is Safari only, FF doesn't support it afaict. I'm not sure why you'd be having problems with selection.modify("move", "forward", "sentence"), can you include the entire test case?
Created attachment 14769 [details]
Please use the attached file
I see. The test tries to modify a non-editable selection.
Marking confirmed per the above comment (there doesn't seem to be any reason to restrict selection.modify() to editable content).