From MozillaZine Knowledge Base

The caveats seems to be outdated:

> "This preference can only be changed by editing channel-prefs.js in the application directory."

At least on FF3.5, about:config can be used to change this setting which immediately reflects in pref.js --Andig 13:41, 30 June 2009 (UTC)

Did you verify that the user_pref() value is actually used? It is my understanding that it is simply ignored and therefore you have to set the original pref() value in channel-prefs.js, but that may have changed. --Rsx11m 23:23, 1 July 2009 (UTC)