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.
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:
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.
来源：https://stackoverflow.com/questions/24489080/firefox-developer-tools-adding-a-new-rule