I'm not sure if this is an Angular or Firefox bug (or maybe I'm just missing something else) because no other browsers are having this problem...

But given a code snippet like:

<div ng-class="{className: true, watch: true, sympathy: true}">

Firefox will generate:

<div class="className sympathy"><div>

You can see this using the following Pen (Angular 1.3.2):
http://codepen.io/joshmatz/pen/Xbmame