That's it folks, Firebug is officially discontinued and as of today, with Firefox v56, it is not working anymore. So the alternative is Firefox Developer Tools.
I tested it and it seems really weak when comparing.
My question is, when you inspect an element, and edit HTML, with Firebug, you could see the changes that you type live as you're typing it, but with the developer tools, you need to finish editing entirely before the changes are applied. Is there any way to make the changes appear live while you are typing them out?