Actually that's not correct. The computed styles pane isn't telling you what font is being rendered. The Firefox Font Inspector is telling you what is actually being rendered on the screen. This is impossible to do with IE and Chrome.
Try it out: Specify a font not on your system. Chrome will show you that font while Firefox will tell you what fallback is being uses.
Try it out: Specify a font not on your system. Chrome will show you that font while Firefox will tell you what fallback is being uses.