Rated 5 out of 5 stars

per Firefox user 6c38bb el mar. 5, 2021 · enllaç permanent

There is a way to "mark all read":
Choose "mark" on the Thunderbird and then choose "mark all read" from the dropdown menu that appears.
The downside (aside from needing several mouse actions) is that a message must be selected for the "mark" button to be active. The keyboard shortcut "shift-c" does not have this restriction.