Weird bug in Firefox 50 & 51

[fahrradflucht]

Steps to reproduce:

1. Open Firefox 50 or 51 (Aurora or Nightly at the time of this writing)
2. Goto http://unsoundscapes.com/elm-flatris.html
3. Click New Game

The Problem

Note that this bug does not always appear. I was able to reproduce it far more reliable in 51.

The game starts to run and I looks like the logic is happening because the score count rises, but the blocks don't get rendered to the screen.

This could very well be a Firefox bug but I thought I report it here first as you might be able to better point out where the issue is caused, then me.