Firefox and Safari seem rather confused about the height of the contents of grid elements.
This nicely coloured grid looks great until you try and fill it with stuff.
This is fine in Chrome, but odd in Safari and Firefox (in the same way)
Chrome 57
Safari 10.1
Firefox 53.0
This is even weirder - it does what you might expect in Chrome, fixes Safari assuming it's behaving differently, and ends up particularly weird in FF.
Chrome 57
Safari 10.1
Firefox 53.0
¯\_(ツ)_/¯