When I'm debugging font-face issues on my websites, I often get confused because I don't really know if it's the web font which is loading, or if it's my locally installed font (and I can't take for granted that every one of my users have that specific font installed).
The "font" tab in the Firefox inspector really helps with this, and I suppose a checkbox here to disable (and/or highlight) local fonts would help debugging fonts even more easily.