Thought..
"Why is Browsercade not working in Firefox?"

Further Thought..
"It SHOULD work.. Shoebox works, and it's practically the same script.."

Further thought..
"Uhoh..!!"

-=-=-

So, um, something's very broken on the Firefox side of things.
Chrome, Fine.
Edge, Fine.
Edge Chromium, Fine.
iOS Safari, Fine.
Firefox.. Not so fine..

It'll load perfectly the first time, but if you close the browser and retry, it breaks absolutely.
ShoeboxOfGames.com, and Browsercade.com equally, and for seemingly the same reason.
It seems to store the cache.manifest, and the index, but none of the important .js files, and then it checks the cache.manifest, sees that it "should" have the important .js files cached, tries to load them, and fails because it doesn't have it cached!!

Probably going to spend the next day or so trying to figure this one out.
Grrr..

Still, at least the font works!