Are you one of the many Add-on authors directing your contributions to Mozilla Foundation? If so, thanks so much! Also, to make setting up your contribution module easier, we’ve put together this “thank you” text:
Thank you for your donation to the Mozilla Foundation. Your donation
will help Mozilla continue to develop open technologies and standards that help keep the internet open. Learn more about us at Mozilla.org
Thanks again to the Add-on authors who have opted to pass your contributions!

It sounds like this text comes up only after making a contribution. If that’s the case then I wonder whether this is so much of an improvement – the user is still thinking that he is donating to the extension.
Unfortunately, I cannot try this, AMO is having out of memory errors again…
There’s a bug filed to make it clearer in AMO, but we did provide text explaining that the contribution was going to the Foundation in the extension description. (the first link in my post).