Are you displaying the classic menu bar? That takes up some space. Tap the Alt key or press F10, then View > Toolbars > Menu Bar.

It is possible using the Stylish add-on (or by manually adding a style rule to the userChrome.css file) to change the height of various bars. However, it can take quite a bit of experimentation to get it working, so I will leave the detailed exploration to others.

Add extra empty toolbars if you want to see more of the persona.
You can create extra toolbars to get extra space via View > Toolbars > Customize
You have to put something on a toolbar (drag a Space item onto it) before closing the Customize window because empty toolbars are automatically removed.

See http://kb.mozillazine.org/Toolbar_customization