Why Can’t I Open A New Tab By Double-clicking On The Tab Bar?

Because Mozilla disabled this feature, instead double-clicking on the tab bar maximizes or un-maximizes the window. However, some people have said by turning off Tabs on Top (see Can I Move My Tabs?) the double-click functionality returns. Others have said that it will not work regardless if Tabs on Top is turned off. This seems to be the case with most Windows Vista and 7 users and some Mac and Linux users as well.