Firefox developer tools adding a new rule?

Question

I decided to try out the built in Firefox Developer Tools. Seems to be nice, but...

I can\'t find out how to add a new rule to the CSS from Inspector/Rules. By that

Answer 1

Update:

As of FF 33.00 it is now possible to add new CSS rules using the inspector!

---

It is not possible to use the inspector to create new CSS rules with out a browser add-on (see comment below).
Just use the style editor. Type anywhere to make a new rule: