In Firefox for Android, there are hot topics as a list at the top and as a box in the middle. See https://support.mozilla.org/en-US/products/mobile
It's a regression from bug 882941: "At the same time, in the mobile version, please show all of them in the list"
Thanks for finding this out. I fixed this by making Hot Topics not visible. We can keep it that way. No need to make any change in the code.