Same.
Several blogs to upgrade.
Manual upgrade - no problems.
Auto upgrade caused this symptom (loss of visual editor).
Re-install automatically would not complete.
Clearing Firefox cache didn't help.
Many Thanks to rfuchtman for solution above.
A couple of things to add:
TinyMCE folder is found in /wp-includes/js/
(mine contained 11 files instead of 8 in fresh download; 3 extra .php files, I tried just deleting those but it did not seem to work, however if anybody else tries that note next step was also necessary; logout and in again.)
And, I could not see that replacing the files had fixed my problem until I logged out of admin and back in again.
BTW most plugin auto upgrades failing too, so quite a few manual plugin upgrades to work through.
Not the smoothest WP Release upgrade ever, but nothing to rant about - it's a walk in the park compared to a Magento upgrade!!!
Phil.
wwww.use-ip.co.uk