Cant find the "Display CSS per element" (don't know, what it was actually called) function anymore. To see what css is applied to an element and where to find that css was the most useful function for me. Sad. :(

UPDATE: Thanks for the info chrispederick! Its so sad, that obviously Firefox itself crippled this once awesome add-on.
My five stars are in remembrance of the "Web Developer" we used to know. :(