Not just that, but in an earlier release code was shipped to set the pref for users who already had the file, so that anyone who already had one of these files would not observe any behavior regressions.
I can't claim credit for it; I hadn't even realized it had happened until I dug a little bit into the history while reading this thread. All I knew was that my userChrome.css was still working fine... ;)
And yes, file I/O was the issue.