How to make prefs.js mods survive restarts?

[Charlie Zender]

I have added to my prefs.js the line

user_pref("dom.disable_open_during_load", true);

but when I shut down mozilla 0.9.4 and then restart the prefs.js
file is automatically rewritten and no longer contains my custom
mods, i.e., the above line. How does one make custom mods to prefs.js
"sticky", so they survive restarting?

Thanks,
Charlie