This is must have extension. It would be even greater if it could work in tandem with ColorfulTabs.

Just to remind older comments. One can hide FireFox tab bar and Side Panel title with userChrome.css file like that:

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

/* to hide the native tabs */
#TabsToolbar { visibility: collapse; }

/* to hide the sidebar header */
#sidebar-header { visibility: collapse; }