Well, you could try to file a bug report or something. But I bet you they'll just say that this is something that belongs in an extension. And I'd agree with them, as it's just a convenience feature that most people don't really need. You can already walk through the tabs anyway using ctrl+pageup/pagedown or jump to specific tabs with alt+1-9. You just don't get a popup for it.
Yo might want to take this up at the
mozillazine forums if it's that important to you. The people there would be more knowledgeable about this specific subject.