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 I mean ".my-new-rule" not just the "element" rule that is open by default.

FINAL EDIT: This is now possible with the latest Firefox update.

Answer 1

Update:

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

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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:

Answer 2

As of Firefox 33 (in nightly right now https://nightly.mozilla.org/), this works.

When you have a node selected in the inspector, you can click on any of the selectors in the "Rules" sidebar to edit them.