Perhaps this has highlighted a problem in the Metal render backend? Firefox generally sticks to the OpenGL standard (using ANGLE as a workaround where only DirectX is available on Windows) whereas Apple has their own APIs.
For example, Firefox seems to make use of the more proprietary Core Animation API got certain tender tasks, which no other OS needs special code for. If the Core Animation API or the code calling it has a bottleneck, you'll only see this problem on macOS.
It could also be a consequence of the way Apple optimises their laptops. On my phone and laptop, the websites hammers one single core (about 87% CPU usage on my laptop here). Apple has optimised their CPU power curves (on Intel, at least) to turbo high and turbo quickly, making actions like loading a website and opening an app nice and snappy. The design of the laptops doesn't allow these high turbo speeds for more than a few seconds, so CPU performance drops after a few seconds. For most use cases that's fine, but I can see a website like this running into that problem. Same thing is happening on Chrome, which is snappier for me but also more CPU intensive.
Edit: actually, after trying Firefox on my hackintosh (7700k, gtx1080), it's more likely to be hardware related. I have no trouble whatsoever with the website on any browser on my desktop, even the outdated copy of Safari works fine.
It runs very smoothly on Chrome + Windows with a dated computer.