I love Firefox but some JS heavy demos etc. just don't work fast enough. I hate that I know exactly why. It's because they were only ever tested on Chrome during development.
Interesting, I haven't run into it as upto now and I was checking some pretty cool and intense CodePens earlier.
I'll look into some threeJS demos to see how it goes (ex. https://bruno-simon.com/ ). Are there any other specific JS demos you've had a bad experience with?
For me a good example have always been https://kangax.github.io/compat-table/es6/ which rendered very poorly on Firefox. I see it's better now, but still, try to hover over the table (do couple of mouse movement, some circles or something) and see that it's not as responsive as Chrome is.
Another example is iD[0] Editor on https://www.openstreetmap.org/. But there, to be fair, it's also slow on Chrome but I feel it's a bit slower on Firefox.