So you use Google Reader and Firefox, but you don't want to visit reader.google.com from time to time to see what's new. Google Reader Notifier is a Firefox extension, developed by Mark D.B.D, that shows you how many new posts are in Google Reader. You can also see the number of unread items for each label and get alerts if there's something new. Of course, those numbers aren't very helpful because +100 can be anything from 101 to a googol or more.
For Mac users, there's an even better solution: a software called the same, that has sound alerts, shows the titles for the latest posts and sits nicely in the the menubar next to Google Notifier.
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