Just followed the solution from mikebz5. (after unlocking and rooting)

Downloaded the Autorun manager, executed the app, enabled system settings, used advanced options, searched for google framework, clicked on it, disabled  com.google.android.gsf.update.SystemUpdateService, and the nagging was over.

Thanks for the great support!

Thanks for the info freakshow999 !!!
I will try the solutions tomorrow.
I searched for "firmware update" messages to find a solution, but i had to search for OTA (noted for nexttime).