mathjax doesn't render for firefox

![image](https://user-images.githubusercontent.com/1280389/39651988-00d64e0a-4fba-11e8-8da8-516eaebc2cac.png)

originally reported in https://github.com/plotly/streambed/issues/2674, reported again in https://github.com/plotly/dash-core-components/pull/194