[Project_owners] how to initial some variables

[Amin]

i have some variable in my extension , i want to set them first time after
instalation

    var prefs = Components.classes["@mozilla.org/preferences-service;1
"].getService(Components.interfaces.nsIPrefService).getBranch("
amin.eft_PhProxy at gmail.com");
        prefs.setCharPref(id, sData);



where i must put code like this to set them only one time ?
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