With the release of FireFox version 30 we have seen a large number of users reporting that Citrix web interface is no longer working.
It seems Firefox 30 has changed the way it handles addons, and specifically it has broken the way Citrix Receiver handles launching.
Firefox does not give you any error or warning, it simply does not launch the application when you click on it.
The fix is actually quite easy. In Firefox in the address bar simply type ABOUT:ADDONS hit enter, Go to Plugins find Citrix Receiver and change from "Ask to Activate" to "Always Activate".
Go back to web interface and try an application and it will work now.