In Firefox, console.log is not showing anything

Reinstalling the Firebug plugin solved the issue. Now it appears in the Firebug console (not in Ctrl + Shift + J).

For some previous versions of Firefox

I had the same problem with a different root cause.

Hit Ctrl + Shift + K to open the full console. In the second row from the top there are tabs - Net, CSS, JS, Security, Logging, and Server. Hit the triangle to the right of "Logging" to open the dropdown and make sure that at least "log" is on.