The tiny little detail that drives me nuts is that, in Chrome, to disable a CSS rule, the checkbox is on the right side of the rule, not the left.

The problem is that I can't visually tell which checkboxes match which rules, because they're too far away. Even worse, the checkboxes only show up on hover, and disappear when you hover just one pixel to the right of them, because the hover area is set wrong. (At least on OSX.)

It's just all the little details the Firebug gets right, and Chrome doesn't seem to, but day after day, they add up.