Long, hard search produced a result. Lots of folks seem to have been having this problem, in completely different versions of WordPress (2.0.x through 2.3.x), for over 11 months. Dozens of solutions appear, from disabling gzipSupport inside the tiny_mce_gzip.php file to stopping the Scriptaculous javascript library from loading.
None of those worked for me.
What worked for me (WordPress 2.3.2) was the following solution: http://wordpress.org/support/topic/123892 .
(I named my function compat_realpath() and did some reformatting, but it's essentially the same as muchachocolo's code). Replacing calls to realpath() with calls to compat_realpath() and reloading 'Write Post' produced a WYSISOWYG editor toolbar and no more Javascript errors on the Firefox console.
Good luck to other folks.