Websites like ipv6-test.com and test-ipv6.com say that my browser is using IPv4 by default.

This happens on Firefox and LibreWolf. I have tried creating a new profile without add-ons as well. Ungoogled Chromium uses IPv6 by default. If I go to ifconfig.io using Firefox, IPv4 is displayed. But if I use Ungoogled Chromium, IPv6 is displayed.

Is there a way to force IPv6?

I am not bothered by this, but using tools like ping6 and traceroute6 assures me that IPv6 routing is faster for, at least, one of my ISPs.