I couldn't find any option for selecting the Firebug extension in the JavaScript gear option, and I didn't want to add external links/libraries, but there is another simple solution.
None of the above solutions worked for me, since I needed an interactive console to be able to dynamically set a variable when testing reactivity in Vue.js.
For future reference: the jsfiddle-console from answer was exactly what I needed when teaching a class on JavaScript. However I found it to be too limited to be of any actual use in this situation. So I made my own.
Maybe it will serve anyone here.
Just add the CDN-version to the resources of jsFiddle:
I might be late to the party but just wanted to mention that JSfiddle has just released the new console feature. Just turn it on in the Settings if it doesn't work for you.
Still in beta but hey... no more annoying workarounds.