The "save this login" prompt using bogus information
When I go to log into sites, I get a prompt from Firefox to update the stored password, even though I'm logging in with the autofilled password. When I click to show the password they want to update with it pops up as "test". I don't get this problem with Chrome and Edge, but it does happen on FF with different computers.
Some websites modify the login form when you submit the form and copy the actual values to hidden fields. If Firefox notices such a change in the fields then you get a dialog to update the stored username and password. In such a case you can create a login block exception to suppress this update dialog.