On the box model the text indicating content, padding etc is white. It is completely illegible.
When you inspect a element and go to the box model and hover/click on the different regions of the box model it shows what region the color indicates, for example content, padding,border etc. In Firefox 37.0.1 the text which show the name of the region is White, it is completely illegible and impossible to read. Why would you make the text white? Please revert the change.
Try to see if there was an option that was changed in the tool:
Else please contact stackoverflow.com and tag mdn, there is a dedicated team for more information.