Go to Tools > Add-ons > The Themes pane to uninstall Personas.

Can't see the Menu Bar (File, Edit, View, History...Help)? Hold down the key and press the following letters in this exact order: V T M The Menu Bar should now be displayed permanently, unless you turn it off again (View > Toolbars). Turning the Menu Bar on and off is a new feature in version 3.6.

Also see Restore the Menu bar in Firefox and Use themes to change the look of Firefox

How do I remove a recently posted personna?

Uninstall: In Add-ons > Themes, you can click on a Persona then click Uninstall>

Choose another: In Add-ons > Themes, click Default (or another Theme or Persona), click "Use Theme".

See: http://support.mozilla.com/en-US/kb/Using+themes+with+Firefox#Managing_themes_and_personas