Disregard my previous pre-edit post. Rethought what I needed to do. This is what I'm currently doing: https://developer.mozilla.org/en/XPCOM_Interface_Reference/nsIScriptableIO

downloadFile: function(httpLoc) {
    try {           
        //new obj_URI object
        var obj_URI = Cc["@mozilla.org/network/io-service;1"].getService(Ci.nsIIOService).
                        newURI(httpLoc, null, null);

        //new file object                               

        //set file with path
        obj_TargetFile.initWithPath("C:\\javascript\\cache\\test.pdf");
        //if file doesn't exist, create
        if(!obj_TargetFile.exists()) {
            obj_TargetFile.create(0x00,0644);
        }

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


        // with persist flags if desired
        const nsIWBP = Ci.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);
        return true;
    } catch (e) {
        alert(e);
    }
},//end downloadFile

As you can see the directory is hardcoded in there, I want to save the pdf file open in the active tab to a relative temporary directory, or anywhere that's out of the way enough that the user won't stumble along and delete it. I'm going to try that using File I/O, I was under the impression that what I was looking for was in scriptable file I/O and thus disabled.