privacytests.org shows how Librewolf privacy testing scores compares to others.
Brave, Librewolf, Mullvad, and Tor hide the user's screen dimensions, whereas Firefox and Chrome don't. I'm curious, how often does that break things? I imagine that quite a few sites use this for choosing how to position things with JavaScript. Certainly not a majority, but with how many websites an average techy visits on an average day, it might still be somewhat frequent that you get ill-fitting content.
Do you use Librewolf, or does someone else use it who can tell?
Pretty much never. They hide it by changing it. This results in a margin around your view port that is fitted to one of several common dimensions. Since they are common, nearly all websites look okay in it.