Hey,
after yesterdays troubles, I ran into a very similar problem. I want to post an answer in my support thread, but when I hit "submit reply", there is the turning-circle-animation in the URL bar for about one second, but nothing happens and the reply is not saved to the forum.
I tried posting the same reply to the testforum, and it also didn't work. I used some editing in the reply, mainly two quotes and some lines are bold but others are not. A usual reply, without bolding, seems to work.
Firefox 46.0.1 on Win10Pro