[CentOS] defaultPref() does not work in firefox

I'm needing to force some settings system wide for Firefox.

However, I am finding that using lockPref() in firefox.js silently 
fails... and causes all directives that come after it in the file to be 
ignored.

An strace shows that it reads the firefox.js file in its entirety and 
then goes on about its business reading other config files and then 
terminates for no apparent reason.

Is anyone else observing this?  I'm running Centos 4.4 x86 with 
firefox-1.5.0.9-0.1.el4.centos4.

Thanks,
Steve