first, make Firefox default browser then open Firefox settings scroll down click private browsing then enable open links in a private tab

hope this helps you. 😊😊

Thanks you very much, but I want the link in a new tab in the same browser not in the private Browser.

I don't use Startpage on mobile, but on desktop, you can specify that individual results open in a new tab using their Settings page:

https://www.startpage.com/do/settings

Does that have any effect on mobile?