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NEW in 2.2: The unread message count is also displayed on the extension button, and clicking on the extension button will directly show the contents of your inbox in a panel. There is a "mark as read" button to mark all messages as read.
I'm probably talking to myself here since it appears nobody except me has been on this page in years, but I figured I should mention that (contrary to my previous comment) the add-in still works just fine, and I've been using the Firefox version daily (minutely?!) for months without issue.