Persona images are meant to decorate the Toolbars and Tab-strip at the top of the window and the Statusbar at the bottom of the browser window, they aren't meant to decorate the "content" or web page section of the window.

If you want to see more of the Persona image, add some Toolbars to see the entire height of the persona image = 3000px by 200px (I don't think the full width of the Persona image can be seen on anything except a wide screen TV display).

Firefox comes with 3 Toolbars, the Menu bar, the Navigation bar, and the Bookmarks toolbar. It may vary depending on your operating system and whether you have Large or Small toolbar icons selected, but on my system I need to have 10 toolbars showing to see the full height of the Persona I have selected. YMMV

View > Toolbars - Customize Then hit the Add New Toolbar button, name that new toolbar (I named them Spacer 1 -7), and then add a single Space to each new Toolbar. Repeat until you have 7 more Toolbars. When you're done - hit the Done button.

You can create one toolbar (PersonaFiller) with a single Space element and use code in userChrome.css to set a min-height for that toolbar. If you do not want to see that toolbar to get more website space then you only need to remove one check-mark via the right-click context menu

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */

#__customToolbar_PersonaFiller {
 min-height:100px !important;
} 

Add code to userChrome.css below the @namespace line.

See http://kb.mozillazine.org/Editing_configuration#How_to_edit_configuration_files