STRs: - start a toolbox targetting an addon (via about:debugging#addons for instance) => "toggle split console" button is missing and cannot be enabled in the DevTools options. See http://searchfox.org/mozilla-central/rev/aa1343961fca52e8c7dab16788530da31aab7c8d/devtools/client/definitions.js#469 We explicitly disable the "toggle split console" button is the target is an addon. The ESC key shortcut works and toggles the split console as expected.
I can't think of a reason why this should be disabled for addons
For the record, this was originally added in Bug 964819. But since the key shortcut works, I also don't see any reason to hide the icon.