For ajax form post you might have problem regarding getting the tinyMCE value in form post. To resolve this problem you need to get the value of timyMCE before the form submit and send it to ajax post medhod and get it from there. Like I have an event message with tinyMCE and I was submitting it with ajax and was not getting the value of it. So I came up with the following solutions.
Firts I took the value of event mesassage
var event_message = tinyMCE.getInstanceById('event_message').getBody().innerHTML
then send it to the ajax data:
by this way I get value of tinyMCE