I've been having that for weeks. Disabling your Firefox extensions is supposed to help in some cases, but I haven't seen any difference.
I have tried using traceroute to try and shed some light on the matter.
derpicdn.net connects in 9 hops.
derpiboo.ru and derpibooru.org both time out, with problems showing up after hop 11, though both can be successfully pinged with times on the order of 330ms.
Unfortunately, I don't really know what to make of these findings, given networking is one of my weak areas when it comes to computing.
Also, I've had to make multiple attempts to post this reply. It keeps landing me on the home page rather than the thread with the reply posted.