I think it's a good idea to mitm yourself and look at what exactly your browser is up to. We should be careful about just accepting and repeating hearsay when such claims are pretty easy to verify yourself.

https://sizeof.cat/post/web-browser-telemetry-2025-edition/

As for webapps breaking in Librewolf, IME those can be fixed by selectively unblocking canvas (or whatever) for the site in question.