This example demonstrates the problem: http://codepen.io/anon/pen/xbmxjz

The button toggles between flex-basis: 0%; and flex-basis: 0px; on the element that has overflow content. In both Chrome 41 and Safari 8 toggling the class has no effect. In Firefox, the height of wrapper overflow: auto; element grows to the height of its content when the basis uses 0% instead of 0px.

This can probably be boiled down to an even smaller test case to put into a concise bug report. Seeing that Safari and Chrome have no problem and that 0% == 0px == 0 in all other CSS attributes this doesn't seem like correct behavior.