Actually, in Safari 3 (3.0 or 3.1) there's no need to activate the Debug/Developer menus, since there's an Import Bookmarks feature right in the File menu. You'll want to export your bookmarks from Firefox first, which you can do in Firefox's Bookmarks Manager (just look in the menus when the Bookmark Manager is in the front - I know there's a command there). Make a note of where you saved this file then navigate Safari to it when you import.