How do I set set a startpage as my default?

this information is correct as of early 2020

Chrome

https://support.google.com/chrome/answer/95314?hl=en

Firefox

In Firefox you can set your homepage and new window page: https://support.mozilla.org/en-US/kb/how-to-set-the-home-page

But this will not open a new tab with this page! Unfortunately a extension is needed to set this functionality, I recommend https://addons.mozilla.org/de/firefox/addon/new-tab-override/.