Looking at the source it's using three.js 62. That's from 7 years ago. This explains a lot.

What's happening is that back in 2013 Chrome and IE used the non-standard 'MouseWheelEvent' while Firefox used the at-the-time-non-standard-but-is-the-standard-now 'WheelEvent'. Chrome 31 switched from MouseWheelEvent to WheelEvent[+], but that came out in Nov 2013, so when this was written it required some additional code to do cross-browser mouse wheel events (jquery.mousewheel.js in this page's code). Presumably something in that jquery plugin is still trying to do browser API detection and fires the old Firefox wheelEvent rather than the new standard version.

It's a shame no one from Google went back and maintained the code for this really, but we all know how Googlers feel about maintaining old code. :)

[+] Around 2012/2013 there were actually three competing mousewheel events - mousewheel[1], MouseWheelEvent[2], and WheelEvent[3]. XKCD 927 applies here (https://xkcd.com/927/).

[1] https://developer.mozilla.org/en-US/docs/Web/API/Element/mou...

[2] https://developer.mozilla.org/en-US/docs/Web/API/MouseWheelE...

[3] https://developer.mozilla.org/en-US/docs/Web/API/WheelEvent

EDIT: Looking at the source of the plugin it also checks for "DOMMouseScroll" and "MozMousePixelScroll". I'm glad we have a standard for this now. Jeez.