The indentation for one. Notice that the second comment is an answer to the first, but is on the same level of indentation. Following discussions like that is highly annoying.

Weird: While grabbing the phone to answer your question I noticed that "threads", i.e. the view of my comments, works fine. Just the comments below submission links are broken.

Compare to Chrome: http://m.imgur.com/Wpusprq

Note that I do use uBlock in Firefox. Now that I think about it, that might be relevant as well. But then again, why would that break the submission view and not the "threads" link?