As purecss has fixed the problem (v0.6) by implementing it in every browser, this answer is obsolete.
Previous answer:
Your problem is that PureCSS is using -ms-display: flex
in Internet Explorer and -webkit-display: flex
in Webkit Browsers. Opera, Firefox and (obviously) older IEs don't get this solution.
To get it working in Firefox (20+) and Opera you could apply the following in your stylesheet:
.pure-g-r {
display: flex;
}
Further information: http://css-tricks.com/using-flexbox/
Here a example using your fiddle: http://jsfiddle.net/f3YNe/12/