Librewolf supports letterboxing as well, though the setting might be disabled by default

Agreed. That’s a really bold claim to make without citing (credible) sources

+1

…it was DNS

If you have a Walmart card, there’s a similar annoyance with the Capital One app on my phone. The screen to pay your bill scrolls up and down just fine, so you can scroll down to pay. The screen to redeem your collected points for cash back? Yeah, no. The screen doesn’t scroll, so I can’t get far enough down to swipe the submit widget.