When Thunderbird loads it loads a default welcome web page that includes a general request for donations. Personally I have not seen it in years, unless I actually try and load it. But you can disable the loading of the start page, simply by disabling the option "when Thunderbird launches, show the start page in the message area." You can even change the page that is loaded to something of your choice. I know some folk load images from the web as the start page.