I'm not sure if this is because of the new major update to Head-Fi recently, but since then I have had some problems with multi-quoting and getting the reply box to show up properly.
Allow me to post some screenshots.
Upon clicking on a thread I sometimes get this layout (note there's no multi-quote option and the reply box is in plain text, no formatting options available what-so-ever)
Refreshing the page, and ignoring the new post, everything goes back to normal for me (note the multi-quote option is back and the reply box is now in rich text, with formatting options available)
I'm using Firefox 23.0.1 on OS X (10.8.4).
Edited by miceblue - 9/5/13 at 9:33pm