@rahul286 and Alphawolf... I don't really know if this is a bug, or even an important discrepancy... plus, I haven't seen this error on my sites. In fact, when I go to an edit page and hover over the media buttons the tip pops up just fine with no complaints from FireBug; still, this issue may only manifest when certain other JavaScript libraries or plugins are loaded... or when someone's plugin is attempting to call the tTips function for its own purpose.
The code in common.js which defines the tTips function doesn't precisely match the recommended aliasing method in the link I posted above.
common.js line 43:
}(jQuery));
the recommended aliasing method from the jQuery team:
})(jQuery);
Try moving the ")" and see if that corrects the issue (without breaking anything, of course). If so, please post something here... and if you can, file a WordPress bug report.