showmodaldialog

I made a small calendar popup in Javascript. Very simple, using the Calendar control from ASP.NET. I call the popup window with showModalDialog. In the modal window, changing the current month of the calendar causes problems because of the postback, and I found in several places that the solution is to put: <base target="_self"/> in the head part of the aspx file. Everything works great... except for one thing, and only in Google Chrome. To get back the selected date, I set the returnValue of the popup to the date selected in the calendar. In IE and Firefox, it always works. In Chrome, however

I was developing a GreaseMonkey script which used window.showModalDialog . But before finishing it, I have discovered that Firefox 29 warns: Use of window.showModalDialog() is deprecated. Use window.open() instead. For more help https://developer.mozilla.org/en-US/docs/Web/API/Window.open But the problem is that window.open needs UniversalBrowserWrite privilege in order to open a modal window using window.open . Then, why is window.showModalDialog deprecated? Is there any API which doesn't require privileges? Note : I don't want a fake modal dialog (like jQuery's one), I need a real modal