Comment 56 for bug 481541

Found it.

http://lxr.mozilla.org/mozilla/source/dom/src/base/nsGlobalWindow.cpp#2415

I know zilch about nsGlobalWindow.cpp but it seems that wrapping lines 2454 and
2461 with appropriate code to check for the existence of a pref just might do
the trick. Perhaps line 2438 could also be wrapped.