‘Docky Unread Count’ for Thunderbird adds an unread e-mail count badge onto your Thunderbird icon in Docky.
The add-on uses the Docky Dbus API to add the badge so there’s nothing to install or compile in Docky itself – it works just using the Thunderbird extension and the api. Pretty awesome!
Right now the add-on only displays an unread badge on the icon. In later releases it may also gain right-click options such as a list of unread mails (similar to the GMail addon) or quick links to compose, inbox, etc.