one of the downsides of yahoo systems is that authentication is set not to allow autocompletion! It's true that it asks the password only once every 15 days but it might be annoying when you often delete your cookies
i've just deleted my cookies after 4 months and now i decided to fix this: a quick google and solution found in the ubuntu forums
basically you can
- install a greasemonkey script: http://blog.monstuff.com/archives/images/AllowPasswordRemembering.user.js
- install a firefox addon http://forum.addonsmirror.net/index.php?showtopic=621