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
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.
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: