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.