It is a Firefox extension where the user can tag any website. Whenever a link pointing to that website is found, it will be decorated with a relevant icon.
The tags are saved to local storage. Much of the functionality comes from Mozilla's example extension 'anotate-page'.
The history is useful since it turns visited links purple*. However, the history remembers every single visited page and has to delete them after some time. Therefore it is not useful over a timespan of years.
(*) In recent years this is no longer true by default and must be forced manually in settings or with an extension
'seen' icon created by Pelin Kahraman from the Noun Project