This was uncovered by the fix in bug 890555. As far as I can tell, this line: http://hg.mozilla.org/mozilla-central/file/6cd6960ea7f6/toolkit/devtools/server/tests/unit/testcompatactors.js#l14 is trying to do something that isn't supported by the ThreadActor: adding a debuggee to a thread actor in a detached state. Ideally this should be fixed ahead of landing 890555 so that we can fix the bug in head.js without disabling too many tests or causing perma-orange. I noticed that simply removing this line removes the error and assertions in relevant test files still pass. Of course, I only looked at this briefly and this could be a red herring. Nick, would you be able to comment on how to fix this test, or if I've misunderstood the problem? Thank you.

In 890555, we discuss the central problem: we're calling addDebuggee when no Debugger has yet been created - so it has no effect.

Let the thread actor handle its global itself. https://tbpl.mozilla.org/?tree=Try&rev=63e17a83423a