The browser your employer ships only currently has partial support for flex box. Why do Mozilla employees insist on being misleading about Firefox's standards support? Flex box is not ready for mainstream use. Not just because of 'old browsers', because of your current browser.
https://bugzilla.mozilla.org/show_bug.cgi?id=702508 this won't be in the stable release for a couple of months.
See http://caniuse.com/#search=flexbox for other issues