1. Go to "Customize..." 2. Remove some item from the toolbar, e.g. the useless flexible spaces 3. Restart firefox 4. Whenever you open a new window, this appears in the browser console: TypeError: this._prefs is undefined urlbar_XBL_Destructor chrome://browser/content/urlbarBindings.xml:146:9 insertWidgetBefore resource:///modules/CustomizableUI.jsm:1290:5 buildArea resource:///modules/CustomizableUI.jsm:848:9 registerToolbarNode resource:///modules/CustomizableUI.jsm:765:9 registerToolbarNode resource:///modules/CustomizableUI.jsm:3176:5 toolbar_XBL_Constructor chrome://browser/content/customizableui/toolbar.xml:48:11 https://searchfox.org/mozilla-central/rev/97cb0aa64ae51adcabff76fb3b5eb18368f5f8ab/browser/base/content/urlbarBindings.xml#145 Regression window: https://hg.mozilla.org/integration/mozilla-inbound/pushloghtml?fromchange=03e5401d560c1a9864bcdff7c4e835fa28575afc&tochange=6cff7451155d9d1bcdcfad795bb79ec8b42a6d06

We are destructing a 'urlbar' binding while creating new windows? Why? :-/

(In reply to Florian Quèze [:florian] from comment #1) > We are destructing a 'urlbar' binding while creating new windows? Why? :-/ The node gets reparented if you customize things such that CUI thinks it needs to occur earlier in the DOM tree than where it is placed by default. Reparenting nodes destructs and then reconstructs bindings.

Comment on attachment 8950870 [details] Bug 1434045 - avoid errors when XBL destructors fire without the constructor having fired, https://reviewboard.mozilla.org/r/220116/#review226012 Thanks!

Pushed by gijskruitbosch@gmail.com: https://hg.mozilla.org/integration/autoland/rev/46cd93930b4e avoid errors when XBL destructors fire without the constructor having fired, r=florian

Doesn't seem to be severe enough to worry about backporting to 59 this late in the cycle.