It kind of seems like Firefox just behaves differently when some aspects of the font aren't explicitly set? The rest of the text looks OK. I suppose I didn't notice because of how it's rendered in Chrome (which is primarily what I tested this in).
I suspect this might just be an easy fix to explicitly set the font size for the axis labels.
(Please forgive my UI mistakes across browsers, this isn't an area I work in often so I didn't fully appreciate browser differences in rendering.)