Which screen are you calling the "splash" screen?

The screen thats starts up, when i click on ThunderbirdPortable.exe See attachment. I can not find this image in the files. Maybe it is hidden in a .dll or somthing? And if so, which file is it?

Thank for your support.

That is something portable apps put in with their portable apps wrapper, it is not a part of Thunderbird.

I suggest you ask in their support forum.

Can you gif me the link of the support forum?

Thanks.

Portableapps.com

https://portableapps.com/forums/support/thunderbird_portable