There seems to be a small bug in FireFox when you are using percentage based margins and Flexbox. If you are using Flexbox, and you have set a percentage based bottom or top margin, they are not applied in FireFox. I have setup a CodePen to demonstrate the problem and you can track the Bugzilla file. The margin is applied correctly in Chrome, Safari and Internet Explorer.
See the Pen FqGwu by Chris Blackwell (@chrisblackwell) on CodePen.