Had the same issue when I updated to version 3.9. Visual editor would not work at all.
– Changing themes did not fix it
– Deactivating all plug-ins did not fix it
– Clearing cache did not fix it
My host provider helped me fix my issue – our Word Press “Includes” folder had some permissions that were set incorrectly. They fixed the permissions on the “includes” folder (i have no idea how the permissions got mixed up), then re-installed Word Press 3.9.
All is good now!
