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

Any chance you could share your script? I’ve tried a few browser extensions to solve this but they never seem to work on the sites where I actually need them. Your approach sounds way better!

Not relevant, but am I the only one who sees that preview graphic and hears the “Level Passed” music from Super Mario Bros?