I posted this question but it seems to based on broken premises, hopefully the following question is better:
When I use Firefox "manually" I have set it to open a set of 10 tabs whenenever I open a new window Preferences → Home → Startpage and new windows → Custom addresses (my translation) but according to the comments to the linked question, Selenium creates a temporary profile with default data unless a specific profile is given as an argument.
So why does a window opened by Selenium use the default tab set from my personal profile?
I invoke Selenium, Webdriver and Firefox like this:
WebDriver driver = new FirefoxDriver();
In other words, very basic.
I use the 3.141.59 Web Driver for Java downloaded from here https://www.selenium.dev/downloads/