Firefox support

**Status:**
:+1: Ready for merge. :+1: 

**Changelist:**
* Changed from having ```html, body``` hidden and faded in on load to just ```body``` ([here](https://github.com/l1f7/l1f7.github.io/blob/bdd57b1f411c557c7507006c0275e81e8fe6c21a/assets/js/main.js#L555)). For some reason, Firefox doesn't allow or like having the ```html``` element hidden (perhaps it removes the fading-in JS from the DOM, so it can't fade in?). 
* Changed relevant header CSS.

Don't think this should break anything, as it still hides all content visible to the user.