So, it looks like the MathML math is getting rendered super high. On chrome, it appears that rendering is done with MathJax.
I'm not sure sure if this is an unfixable thing with Firefox, but if it is, maybe the page should just use MathJax on Firefox too? For what it's worth, StackExchange Mathematics uses MathJax on both Chrome and Firefox, and renders identically on both.
I'm experiencing the following on Firefox 63.0 on Arch Linux:
https://i.imgur.com/D39YePh.png
https://i.imgur.com/gWCakyj.png
So, it looks like the MathML math is getting rendered super high. On chrome, it appears that rendering is done with MathJax.
I'm not sure sure if this is an unfixable thing with Firefox, but if it is, maybe the page should just use MathJax on Firefox too? For what it's worth, StackExchange Mathematics uses MathJax on both Chrome and Firefox, and renders identically on both.