1. Open about:config and set toolkit.legacyUserProfileCustomizations.stylesheets to true
2. Put this under the Firefox profile:
$ cat chrome/userChrome.css
/* hides the native tabs */
#TabsToolbar {
visibility: collapse;
}
/* adjust navigation bar size */
#nav-bar {
margin-top: 1px;
margin-right: 74px;
margin-bottom: 1px;
}