Looks for btnDumpPlatform.addEventListener in lib/main.js and make it like this:
 
btnDumpPlatform.addEventListener("click", () => {
    const info = {
        "PolyfillService": toJSON(gService, "is"),
        "appInfo": toJSON(Services.appinfo),
        "addonData": toJSON(gAddonData),
    };
    delete info["addonData"]["installPath"];
    const dumpInfo = JSON.stringify(info, null, '\t');
    try {
        const filePicker = Cc["@mozilla.org/filepicker;1"].createInstance(Ci.nsIFilePicker);
        filePicker.init(Services.wm.getMostRecentWindow("navigator:browser"), "Save platform information", Ci.nsIFilePicker.modeSave);
        filePicker.appendFilter("JSON file", "*.json");
        filePicker.appendFilters(Ci.nsIFilePicker.filterAll);
        filePicker.defaultString = "PlatformInfo.json";
        filePicker.defaultExtension = "json";
        if (filePicker.show() != Ci.nsIFilePicker.returnCancel) {
            const fs = Cc["@mozilla.org/network/file-output-stream;1"].createInstance(Ci.nsIFileOutputStream);
            fs.init(filePicker.file, -1, -1, 0);
            fs.write(dumpInfo, dumpInfo.length);
            fs.close();
        }
    } catch (e) {
        alert("Palefill", e.toString());
    }
    console.log(dumpInfo);
});