var gTimer = {
    preferences: null,
    autorun: null,
    timer: null,
    min: 0,
    hour: 0,
    interval: 1,
    nullLabel: true,
    init: function() {
        this.hour = 0;
        this.min = 0;
        this.preferences = Components.classes["@mozilla.org/preferences-service;1"]
            .getService(Components.interfaces.nsIPrefService)
            .getBranch("extensions.gtimer.");
        this.preferences.QueryInterface(Components.interfaces.nsIPrefBranch2);
        this.preferences.addObserver("", gTimer, false);
        // почему здесь gTimer ?
        gTimer.interval = this.preferences.getIntPref("interval");
        this.autorun = this.preferences.getBoolPref("autorun");
        if(this.autorun) setTimeout("gTimer.start()", 100);
    },
    observe: function(subject, topic, data) {
        if(topic != "nsPref:changed") return;
        switch(data) {
            case "interval": {
                this.interval = subject.getIntPref("interval");            
                break;
            }
            case "autorun": {
                this.autorun = subject.getBoolPref("autorun");            
                break;
            }
        }
    },
// дальше остальной код