All threads so far have this behavior: replies to any post in the thread appear at the very bottom of the thread. It is possible, with careful scrolling and good eyesight, to see which post a reply refers to, but it is time-consuming and frustrating. I am thinking the intended behavior would be to have each reply appear right below the post to which it replies, as in other threading apps/pages. If this is a feature instead of a bug, I will change this to a feature request (or make a new feature request).
Tested on Firefox 145.0.2, Linux Mint.