When I visit Yahoo! Mail in a desktop web browser, and I am using the "full-featured" and "newest" Yahoo! Mail interface, I don't see a way to mark all emails in all folders as "read."
The only option that I see is to right-click each folder title and then click "Mark as read." However, if you have many folders, this option is not particularly quick.
On a whim, to see if I could at all decrease the total number of necessary clicks, I tried left-clicking each folder title while holding down the ⌘ command key. But this method did not select more than one folder at once, as I had hoped.
Is there a way to mark all unread emails as read? Does a relevant keyboard shortcut exist?