Apparently, FR is being slow in returning confirmation that “Posts” have been accepted. Patience is advised for those who don’t want to be “double-posters” — as I just was... ;-}
Usually, it takes milliseconds for the server to receive the post. The time delay or even a hang that occurs after you hit Post is usually the time required for the server to send you the modified thread back and for your browser to receive it in sufficient measure to start displaying it (it arrives in parts).
If in doubt whether your post has been received, open another browser and see if it is there. Very often the other session oe browser will be faster than the one still waiting for the response after Post.
I don’t remember a single time when my post that appeared hanging was not already there, unless of course, I lost the connection completely even before posting.