I've been using Firefox for almost 3 years. Organizing them by date added wouldn't do any good if I wanted to search for, say 'IE7'. So I have made up folders by subjects>> For instance, Computer> Browsers> Firefox.
That means I can search by Computer, Browser or Firefox, depending on how I want to narrow the search. If you set up the folders, then open the Bookmarks> Organize Bookmarks, you can drag any Bookmark on the right screen to the folder on the left.
I don't even see an option to sort by date. Is there some reason you think they would best serve you if sorted by date? Wouldn't you then have to remember which date you saved the Bookmark in order to bring it up?