Librewolf[0] is probably the best alternative, drop-in replacement for firefox in terms of privacy. It's essentially (base firefox + privacy patches + privacy policies and configs), and available on Windows, Mac, Ubuntu, and more.

edit: Expect though that some parts of the web won't work with it since WebGL and DRM are disabled.

[0] https://librewolf.net/

have been using librewolf. seems like a good balance of "not mozilla" and usable.