I too am having the same problem…
Mac 10.9.2
Firefox – stuck at Adding…
Chrome – no problem
Safari – stuck at Adding…
…and I believe I have found the culprit. Within the TinyMCE Advanced settings under Advanced Options, there is a checkbox that reads: Stop removing the <p> and
tags when saving and show them in the HTML editor. When this is checked, the problem arises; uncheck it, it ceases.
I hope this helps…
Thanks!