I am using the new flexbox settings to vertically center a row of columns between the top of my page and the start of main content (little gray crosshairs in the screenshot). It works well in Safari, Chrome, even IE. In FF however, the row of columns remains vertically centered, but the columns all condense horizontally so that the little crosshair graphics are pushed closer together (see screenshots).
I've noticed that if I change the header div to be a block instead of flex, the columns are spaced out horizontally in FF (correct), but of course I can no longer vertically center the row of columns in a responsive way. (I could just add padding/margin to push them down, but I want that space the top to be dynamic, based on a user's browser window size.