I’m built a graph in Chrome which looks like this:

But in Firefox it looks like this:

I’m pretty sure the discrepancy is the result of display:table-cell getting handled differently in the two browsers. But I’m not sure how to fix it. I uploaded a live demo here so you can inspect the code.
In order to fix it:
display: table-rowto.graphinline-blockinstead oftable-cellon.series