Scrollbars autohide even with System Preferences → General → Show Scroll Bars set to "Always"

[Wowfunhappy]

In OS X's "General" Preference Pane, there is a setting called "Show scroll bars" with three options (not in the below order):

1. When Scrolling. Scrollbars will hide and show automatically, a la iOS.
2. Always. Scrollbars will always be visible, a la most desktop operating systems prior to the late 2000s.
3. Automatic. The default setting. Acts like #1 if a trackpad is connected, otherwise like #2.

In Firefox Dynasty, scrollbars always hide automatically regardless of which option is selected or what input device is in use.