dropout-progress by Bladt

Keeps track of:
Which season you are currently watching,
which episodes you have already seen in a season, and how far you've made into a given episode.

NB: The extension can only track your progress since installation.
That it, progress you've made before installing this extension will not be displayed.
Unfortunately I don't see this changing in future versions.
However, clicking the dropout-progress menu in the season listings will let you mark a season as watched at a time!