This extension adds a button in the footer of each entry. Clicking this button will mark all previous entries belonging to the current feed, as read.
The goal is, when going through a very long list of entries without reading them all, to be able to mark as read what was went through, and continue later.
Download the code and copy directory xExtension-MarkPreviousAsRead into the extensions/ directory of your FreshRSS installation.
Then, in the settings menu, select Extensions and enable Mark Previous as Read.