This has to do with the flexbox specification's implied minimum sizing algorithm.
This is a Firefox bug.
Adding min-width: 0; min-height: 0 to #wrapper seems to do the trick:
#wrapper {
display: flex;
flex-grow: 2;
flex-basis: 0%;
min-height: 0; /* NEW */
min-width: 0; /* NEW */
}
DEMO
Original Answer Below
Currently you have overflow-y: scroll applied to #left.
If you also apply overflow-y: scroll to #wrapper, the vertical scroll launches in Firefox.
As to why Firefox interprets the code differently than Chrome I can't say. The rendering engines have their differences. I recently addressed another flexbox interpretation issue between IE11 and Chrome:
More information:
Bug 1043520 - (minsizeauto-fallout) Tracking bug for web content breaking due to new "min-width:auto" / "min-height:auto" behavior on flex items
Bug 570036 - Flexible box does not allow overflow scrolling of children elements without extra markup
DEMO: http://jsfiddle.net/seqgz8qv/ (scroll works in FF)
Scroll not working with CSS3 flex box in Firefox
Flexbox and vertical scroll in a full-height app using NEWER flexbox api
Scrolling a flexbox with overflowing content
