Use flexbox standard syntax

[gugadev]

It seems you guys have forgotten to place display: flex in the css classes.

This is the look under Firefox 45.x with default styles:

After add display: flex:

[wincent]

Thanks for the report! I couldn't repro this on Firefox 49 or 50 (didn't have access to 45), so I downloaded v45 and I can repro.

I suspect most people allow the auto-updater to keep them on a more recent version, but v45 is still pretty recent (April 2016, I think), and the cost of fixing the issue seems low, so we may as well do it.