I found this interesting article about “how to effeciently use some less known Inspectors features”.
I am impressed especially by first two topic, available in Chrome, to Toggling Classes With Checkboxes and “Editing Content with designMode”.
Can we consider to implement something similar?
Toggling classes was actually recently implemented by @pbro (see https://bugzilla.mozilla.org/show_bug.cgi?id=1213767) ! It will ship with Firefox 55.
And regarding document.designMode = "on", it’s actually a document API (https://developer.mozilla.org/en-US/docs/Web/API/Document/designMode) and is already available in Firefox. Just open the Console and type document.designMode = "on"
You know what @ldetomi? That is a pretty cool feature!
I made a WebExtension for toggling document.designMode. It is under review in AMO.
It usually takes a day to get passed the review 
Hmmmm… my addon only works in Firefox Nightly 
Error: No window matching {“matchesHost”:["<all_urls>"]}
I did not ask for that permission.
So… @ldetomi you can have that feature only if run Nightly for now
