why does this JavaScript function stop execution?

I have the following JS function which serves as a first prototype for a mozilla thunderbird extension. The goal is to connect to a server and download a sample file, then unzipping it and storing the contents in the thunderbird profile folder. Now this all works fine, except that the execution of the function stops after creating the zip file on the file system. So i have to restart the function again, in order to get the second part of the function executed which extracts the user.js file from the zip file. Any ideas what the problem could be?

function downloadFile(httpLoc) {
    // get profile directory
    var file = Components.classes["@mozilla.org/file/directory_service;1"].
        getService(Components.interfaces.nsIProperties).
        get("ProfD", Components.interfaces.nsIFile);

    var profilePath = file.path;

    // change profile directory to native style
    profilePath = profilePath.replace(/\\/gi , "\\\\");
    profilePath = profilePath.toLowerCase();

    // download the zip file
    try {
        //new obj_URI object
        var obj_URI = Components.classes["@mozilla.org/network/io-service;1"].getService(Components.interfaces.nsIIOService).newURI(httpLoc, null, null);

        //new file object
        var obj_TargetFile = Components.classes["@mozilla.org/file/local;1"].createInstance(Components.int开发者_开发技巧erfaces.nsILocalFile);

        //set to download the zip file into the profil direct
        obj_TargetFile.initWithPath(profilePath + "\/" + "test.zip");

        //if file the zip file doesn't exist, create it
        if(!obj_TargetFile.exists()) {

            alert("zip file wird erstellt");

            obj_TargetFile.create(0x00,0644);

        }

        //new persitence object
        var obj_Persist = Components.classes["@mozilla.org/embedding/browser/nsWebBrowserPersist;1"].createInstance(Components.interfaces.nsIWebBrowserPersist);


        // with persist flags if desired ??
        const nsIWBP = Components.interfaces.nsIWebBrowserPersist;
        const flags = nsIWBP.PERSIST_FLAGS_REPLACE_EXISTING_FILES;
        obj_Persist.persistFlags = flags | nsIWBP.PERSIST_FLAGS_FROM_CACHE;

        //save file to target
        obj_Persist.saveURI(obj_URI,null,null,null,null,obj_TargetFile);
    } catch (e) {
        alert(e);
    } finally {
        // unzip the user.js file to the profile direc

        // creat a zipReader, open the zip file
        var zipReader = Components.classes["@mozilla.org/libjar/zip-reader;1"]
            .createInstance(Components.interfaces.nsIZipReader);

        zipReader.open(obj_TargetFile); 

        //new file object, thats where the user.js will be extracted
        var obj_UnzipTarget = Components.classes["@mozilla.org/file/local;1"].createInstance(Components.interfaces.nsILocalFile);

        //set path for the user.js
        obj_UnzipTarget.initWithPath(profilePath + "\/" + "user.js");

        // if user.js doesn't exist, create it
        if(!obj_UnzipTarget.exists()) {
            alert("user.js wird erstellt");

            obj_UnzipTarget.create(0x00,0644);
        }

        // extract the user.js out of the zip file, to the specified path
        zipReader.extract("user.js", obj_UnzipTarget);  

        zipReader.close();
    }
}

var hello = {
    click: function() {
        downloadFile("http://pse2.iam.unibe.ch/profiles/profile.zip");
    },
};

saveURI is asynchronous, so you need to set the progress listener on the persist object to know when it has finished. Here's an example of setting a progress listener and later on there's a check to see whether the transfer has finished.