Please run the SQL from attachment 341269 [details] (from bug 432614) on preview.AMO's DB. It enables the "delete user account" feature, so it can be QAed. (Note: This will have to be run on production also at push time). Thanks!
ALTER TABLE `users` CHANGE `email` `email` VARCHAR( 255 ) CHARACTER SET utf8 COLLATE utf8_general_ci NULL; Query OK, 1635209 rows affected (1 min 27.01 sec) Records: 1635209 Duplicates: 0 Warnings: 0
Verified FIXED, as I successfully deleted a user account using https://preview.addons.mozilla.org/en-US/firefox/users/delete, which means this worked.