Multiple instances of the HTML editor cause Internet Explorer to function unreliably: the editor may be displayed with just a tool bar and no text area, or IE may lockup completely. This causes big problems for students taking quizzes with Essay questions, especially if multiple essay questions are loaded in one quiz.
When we originally wrote the essay question at Humboldt State, we did not enable the HTML editor for this reason.
See http://moodle.org/mod/forum/discuss.php?d=54574 for more information about the problems the HTML editor can cause when used in essay questions.
I would suggest either not displaying the editor in quiz essay questions at all: loading multiple instances of the editor on Firefox can also slow down the page load a good deal, or putting in a browser detection function to hide it when the user is using Internet Explorer on Windows.
Perhaps another option would be a config option in the Quiz module to use or not use the HTML editor in essay questions.