If you want to clear all favicons from your bookmarks in Firefox 3, edit the places.sqlite database, which can be found in your profile folder, located in your user's Library » Application Support » Firefox » Profiles » random_text.default folder. You then need to use any SQLite editor to execute these two commands on that file:
delete all from moz_favicons
update moz_places set favicon_id = NULL
Quit the editor and restart Firefox, and the favicons will be gone (this works on every platform). Also, if you want to permanently disable favicons, type about:config in the URL bar, and edit these two properties:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20081020145751326