[Rebelgecko]

If you go to about:config and set "devtools.chrome.enabled" to true, the cmd-shift-j thing should work

[ezconnect]

I also have to do this to make it work on Win10 firefox console (F12)

[wertyyuu]

If some addons DISAPPEARED then you have to restart Firefox, go to addon manager menu, find disappeared addons and disable/enable them.

When I say "disappeared" I mean that addon icon or else is disappeared.

[leemailll]

This is the step required for me to allow Browser Console for input