Thought I'd share a way to hide horizontal tabs in firefox, if you're using vertical tabs. This leaves 4 pixels up top to drag by. if you use the menus, you can still kind of see the if you press alt.
set toolkit.legacyUserProfileCustomizations.stylesheets to true in about:config
make this file
%APPDATA%\Mozilla\Firefox\Profiles\\chrome\userChrome.css
#TabsToolbar { visibility: collapse; }
#sidebar-header { visibility: collapse !important; }
#nav-bar {
margin-top: -18px;
margin-bottom: -4px;
}