It is only read from the default prefs so user prefs can't change it. Ideally it wouldn't be possible to modify it as a default pref as well. Writing it out from the application would allow any user process to write it out since it would have to be written to a user writable location and which would allow user processes to break updating.
Bug 1568994 Comment 3 Edit History
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It is only read from the default prefs so user prefs can't change it. Ideally it wouldn't be possible to modify it as a default pref as well. Writing it out from the application would allow any user process to write it out since it would have to be written to a user writable location which would allow user processes to break updating.