Changing default search engine systemwide

[pk <pkoroau@gmail.com>]

I want to set DuckDuckGo as the default search engine for new users,
and allow each user to switch back to Google, if desired.

But .mozilla/firefox/<profile id>.default-esr/prefs.js hard-codes
browser.urlbar.placeholderName to Google, so setting it in
/etc/firefox-esr/my_settings.js won't work.

The new users are created on demand (LXC), so it is not an option
to set this manually in each user's user.js or similar.

Can I customize prefs.js and where does Firefox take it from?
I cannot find it in dpkg -L firefox-esr | grep js and in
/usr/lib/firefox-esr/{,browser/}omni.ja.
Or is there another configuration file to try?