Youtube Watchmarker
Youtube keeps track of your watch history and automatically marks videos that you have already watched. They however only mark recently seen videos, which is kind of disappointing. This addon therefore keeps track of your watch history on its own, such that not only the recently seen videos are being marked.
How can the persistence of the database be ensured? There is an export and import feature, which makes it easy to create a backup of the history. This backup can therefore easily be archived persistently.
How can I make sure that the database is complete? The automatic synchronization only considers the recent activity. You are thus encouraged to manually initiate a complete synchronization in the menu, which incorporates as much of your history as possible.