Here's a general warning for Firefox users: Saved passwords are not safe if you do not create a master password. If you use Firefox and you want to see for yourself, go to Firefox's Preferences, visit the Security tab, and press the Show Passwords button. This will show all your passwords in plain text for anyone to see.
You can enable a password to protect this (check the Use a master password box, then enter a password), and you will then have to use the password once per session to unlock all the other passwords being stored. This affects both the PC and Mac versions of Firefox.
Why this matters: If you bring your Mac in for repair, anyone working there can see all your passwords by simply launching your browser. Your password my be your Social Security Number at some sites, such as banks or mortgage companies. Ergo, your financial information would be available to anyone who wanted it.
So if you use Firefox, add a master password to give yourself some level of protection.
[robg adds: Safari, Camino and OmniWeb all use the Keychain to store passwords, so this is exclusively a Firefox issue. On the overall scheme of exposures, I don't think it's a huge one -- your machine should be locked if you're away from it and others have access to it. However, it is a concern when sending a machine back for service, or in any other situation where others may be using it without your supervision. Speaking of sending machines in for service, you might want to read this hint if you're concerned about security during the service period.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070119205357687