Fingerprinting works by collecting bits of information about the browser and device to identify users. Couldn’t browsers see when a website gets such info with JS and either prevent or ask permission from the user for the website to make HTTP requests to upload such information to the website. Idk if they do something like this already.
The letterboxing feature has been in Firefox since 2019 - starting from Firefox 67 I think. The preference for it might have been hidden though so maybe it’s just relatively unknown feature - I don’t know if or how visible LibreWolf makes makes it for the user. But regardless, any modern Firefox variant probably has that capability.
In LibreWolf there’s a toggle in settings for it. That’s one of the disadvantages of Firefox. It has so many features but everything in hidden in a config file. They could just create an “advanced settings” menu.
Yeah… It’s a bit hard to balance things like this though, I’ve seen lot’s of folks complain about how their Firefox is apparently “broken” because it now suddenly has this empty margin around web-content seemingly wasting space for no reason - and then it turns out that they have deliberately turned this very feature on. And that is even if the feature is completely hidden - I wonder how many more complaints there would be if options like this are made more accessible.
Only you are responsible for what you turn on. It’s good to have a description of the feature with warnings about the potential inconveniences it can cause (like LibreWolf does) but the responsibility is still on the user.