Nice to meet you Melih
You could add a donation link under the more tab on the Internet Security and firewall. For example when people click on this link it could take them directly to a paypal shoping cart which has a set recomended donation of lets say 20 cents or pence then an option to choose an amount to donate.
I love your software and thought that this would be a good idea to give you guys more money to further develop the product and for advertising your brand.
As long as your not intrusive with asking for the donation I think people will be happy to donate something.
Thankyou!