From Internet Explorer for OS X
In Mac OS X, there are many applications that use dropdown lists (here
showing "Copies & Pages") in places where tabs might have be used. Dropdowns
are space-conserving, and allow for long or numerous page names, but the
user can't see what other pages are available until they open the dropdown list.
Note the box around the controls; this kind of containment is necessary for
a user to understand what the dropdown does. Otherwise, it just looks like
another control, not a navigational device.