Here is experimental patch adding NPAPI support on DirectFB:
https://bugs.webkit.org/show_bug.cgi?id=36171

However, if you are planning to write plugins yourself, you may find it easier to inject your conent into the page with QObject bridge. More complicated approach (but still more productive and likely more CPU-efficient than NPAPI way) is implementing your custom elements inside WebKit.

Could you specify what are you planning to achieve with plugin?

@AliReza-Beytari said:

So, how can I wait for login.php to finish replying ??!!

Wait for the QNetworkAccessManager::finished() or QNetworkReply::finished() signals. These signals are emitted when the reply has finished.

Do you know how to use signals and slots? http://doc.qt.io/qt-5/signalsandslots.html