I went to this thread earlier: Unbalanced Monsters
When I open the thread (or refresh it) it, I’m prompted with a download for the Twitch player applet presumably. I don’t have Flash installed. Probably don’t have to mention that this is bad. I don’t see an attach button of any sort so I inline link to a screenshot below.
Using Firefox Nightly 46.0.1 on GNU/Linux (NixOS).