I think this is for a Firefox addon: Scripts - WebMail Notifier
If you can access your account using Pop3 I prefer PopMan: PopMan
It's free and simple to use. It works well with Thunderbird 2.x, which is what I use.
But if you are accessing your mail from all over using public PCs or a friend's machine on the road, then you may be better off using Firefox portable with the WebMail AddOn on a USB key.