I had a problem on one of my blogs wherein the WordPress visual editor (or TinyMCE) had gone missing with one of the recent updates. Tried disabling plugins to no avail. Finally found a solution that involves editing wp-config.php (which doesn’t seem to have created any other problems, at least not that I’ve noticed yet).

The solution was to add the following line to wp-config.php

define('CONCATENATE_SCRIPTS', false );