I just tested on Mac and Windows 7 and even with tabs on top, double clicking in empty space next to tabs will open a new tab. If that doesn't work for you, I

I just tested on Mac and Windows 7 and even with tabs on top, double clicking in empty space next to tabs will open a new tab. If that doesn't work for you, I would recommend considering it a bug and filing it on bugzilla. It might be an incompatibility with an add-on or a preference setting, but there is no harm in reporting it.