When starting my debug build, I see this among the output in my terminal: { JavaScript warning: chrome://browser/content/places/controller.js, line 940: yield without a value is deprecated, and illegal in ES6 (use 'yield undefined' instead) } This warning was added recently, in bug 885463, FWIW. mak, looks like you added this 'yield' back in 2011, in this cset for bug 681420: http://hg.mozilla.org/mozilla-central/rev/7a8b51e5be41#l1.208 Is "yield undefined" what we want here now?