With the following HTML body:

<h1 Ω="omega">Hello!</h1>

and the following code:

var h1 = $('h1');
console.log(h1.attr('Ω'));
console.log(h1[0].getAttribute('Ω'));

the h1.attr('Ω') part prints "omega" in Chrome but undefined in Firefox. The getAttribute part prints "omega" in both browsers.

This has hit AngularJS, see angular/angular.js#13318 (comment) for more info.

EDIT: @dmethvin says it works in IE 11.