Looks like Firefox does not like the - in dateString.
Replace all occurrences of - with / using a regular expression and then convert the string to Date object.
var str = '02-24-2014 09:22:21 AM';
str = str.replace(/-/g,'/'); // replaces all occurances of "-" with "/"
var dateObject = new Date(str);
alert(dateObject.toDateString());