A summary from its site and known technical details:
no telemetry by default
includes uBlock Origin
has sane privacy-respecting defaults
prepackages arkenfox user.js
relatively well-maintained fork of Firefox that keeps up with upstream
No major controversies AFAIK
As for Windows 7, nobody should really need to install Librewolf anyway on such a device. No device running Windows 7 should have access to the internet at this point. If you are asking about compatibility intending this use case, you have bigger problems to worry about than your choice of browser. If you just need to view HTML files graphically, even Internet Explorer or an older firefox ESR will do.