Edit: And I’ve saved (and discovered) the best last: If you click your username in the upper right corner, you get your Profile and there’s a checkbox option to disable the visual editor.
Just to be thorough:
Clear your cache (though trying different browsers is effectively the same thing)
Does this problem exist in existing and new posts? How about the same for Pages?
Can you view the code for your page and look for the editor toolbar? Mine looks like: