This is Firefox's password manager. For some reason, instead of just being a preference about the display of unrevealed passwords, every password entry has two "reveal" buttons: one that completely blanks it (the first one), and one that reveals it (the second one). We can totally differentiate these because the first icon is so much bigger than the second one! Or maybe I'm going crazy. Nah, that can't be, my psychologist in the air knows.
It follows intuitively that everytime we see two reveal buttons, they should have the exact same functions everywhere, right? Nope! In the editing interface, both buttons dot he same thing of revealing. Stop staring at me, Firefox!