I think it would be cool if I could have somewhere visible that I have donated to Firefox/Mozilla.
- A badge in the browser toolbar. - Some "golden" corner in the browser. - ...
Probably some people with better design skills than me can think of something even fancier.
The idea behind is to promote donations and also increase visibility. I can imagine a lot of people asking me what is this "weird thing" you have in your browser. And then explaining I am using Firefox, and I am also a donor.