There is no link between the generic startup page with it's request for money and you actually doing it. SO their is no way to "stop asking" after you donate.

You are seeing https://start.thunderbird.net/en-US/release/.

It is a static help/information which is not able to detect whether you donated.

But in a future release, still being worked on, we expect to be able to detect that you have donated. Until then, you might ignore that 1/6 of the page?