After reading various posts about this, I disabled (or deleted) all my plugins to no avail, only to finally come across someone saying there’s a checkbox in Profile to disable the Visual Editor! I’m certain I didn’t check it by mistake – did the last update to 3.9.1 tick it by default?
I was having this same issue with a fresh install of 3.9.1. I remember having to do this one other time.
This seems like a pretty obscure and mislocated ‘fix’ option for the issue. I spent a couple hours searching for the fix (most the results were about the ‘visual’ tab not operating correctly), and trying the typical troubleshooting things like deactivating all plugins and reverting to the the default theme. Reinstalled and all that fun stuff. Remembering this would have saved me a boatload of time!