Issue centering on Firefox 57

[7starsone]

Hello,
there's an issue on Firefox 57

I don't know what is the cause, if the issue is in Firefox or not.

You choose to open the popup inline on page load (so, in a hidden div)

var popup = new $.Popup();
popup.open('#mydiv');

when you load the page the 1st time, it doesn't center the popup.
If you load the page a 2nd time, instead, it's working as usual.

Can you test it, please?
Thanks

[7starsone]

For now, I had to do this:

@media (min-width:1025px) { 
@-moz-document url-prefix() {
.popup_cont {
	position: fixed;
	z-index: 99999;
	top: 0 !important;
	left:50% !important;
	margin-left: -400px !important;
  }
 }
}

my popup is 800px in width, hence that margin-left
So, in this case it stays on center on desktop + Firefox, then it also resizes together with the window.

Anyway, I'd be grateful if you tested this issue and let me know. Thank you