From https://bugzilla.mozilla.org/show_bug.cgi?id=465269#c38: > In the same vein, if you customize the toolbar with the widget and drag it > back and forth, you will eventually see horrible errors (in a debug build) > about XPConnect being called on a scope without Components present. I believe > what is happening is that we are leaking a listener. Although the listener > survives, most of his world gets destroyed, leaving him in a screwed up > scope. I'm not sure we actually need to fix this, as it seems hard/annoying > and unlikely to crop up all that often. Although we should probably log a > bug. The error is: ###!!! ASSERTION: XPConnect is being called on a scope without a 'Components' property!: 'Error', file /Users/bwinton/Programming/thunderbird/folder-location-toolbar/mozilla/js/src/xpconnect/src/xpcwrappednativescope.cpp, line 766 I also get it when adding/removing the "Mail Views" dropdown. And to replicate it, it helps if I hit the "Done" button after adding or removing the dropdown. Thanks, Blake.

This assertion is gone.