The complete error info is: A coding exception was thrown in a Promise resolution callback. See https://developer.mozilla.org/Mozilla/JavaScript_code_modules/Promise.jsm/Promise Full message: TypeError: this._searchSuggestionController is null Full stack: _matchSearchSuggestions/promise<@file:///c:/mozilla-source/central/obj-firefox/dist/bin/components/UnifiedComplete.js:937:1 Handler.prototype.process@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:934:23 this.PromiseWalker.walkerLoop@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:813:7 Promise*this.PromiseWalker.scheduleWalkerLoop@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:744:11 this.PromiseWalker.schedulePromise@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:776:7 this.PromiseWalker.completePromise@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:711:7 this.SearchSuggestionController.prototype._fetchRemote/<@resource://gre/modules/SearchSuggestionController.jsm:250:54 this.SearchSuggestionController.prototype.stop@resource://gre/modules/SearchSuggestionController.jsm:172:7 stop@resource://gre/modules/PlacesSearchAutocompleteProvider.jsm:181:5 stop@file:///c:/mozilla-source/central/obj-firefox/dist/bin/components/UnifiedComplete.js:775:7 UnifiedComplete.prototype.stopSearch@file:///c:/mozilla-source/central/obj-firefox/dist/bin/components/UnifiedComplete.js:1776:7 handleEnter@chrome://browser/content/urlbarBindings.xml:890:1 handleKeyPress@chrome://global/content/bindings/autocomplete.xml:479:24 onKeyPress@chrome://browser/content/urlbarBindings.xml:199:18 onxblkeypress@chrome://global/content/bindings/autocomplete.xml:582:8 This error frequently appears in browser console as well as the command line output.

Reproduced the bug on Nightly 42.0a1 (2015-08-03). I'm no longer seeing these errors as of Nightly 42.0a1 (2015-08-05). Tested with Windows 10 x64 (10240), Ubuntu 14.04 (x86) and Mac OS X 10.10.4.