It seems there is a known issue with the latest
version of FireFox browser and the forums, several members have been unable to post or just the text "undefined" showing when you are composing a post or when you hit the post thread button.
Obviously you could use another browser, but if you still want to use FireFox then click this link http://www.oldapps.com/firefox.php
scroll down a little and download the 15.01 version and reinstall it over my current version of FireFox,without uninstalling anything.
This has already fixed the issue for several members.