No Prompt in Firefox Browser Console?

I'm using Firefox for Ubuntu, version 39.0. I'm trying to debug an add-on and would like to run some JavaScript under chrome privileges. According to this page I should be able to do this in the Browser Console.

If I click on Tools -> Web Developer -> Browser Console in the Firefox menu, I get the Browser Console:

Only problem is, I can't evaluate anything. My question is: where's the prompt? Apparently this window should have a prompt. Things I have tried:

• Restarting Firefox with add-ons disabled (safe mode)
• Starting Firefox as a sudo user

Any tips?

You have to enable developer preferences:

"Setting up an extension development environment" MDN page from

• archived by Wayback Machine
• archived by Unified XUL Platform MDN Backup

Thanks to your research it looks like it was the devtools.chrome.enabled preference.