Disabling Javascript or even just third party scripts does lead to major breakage, but reporting spoofed values for identifiers like Tor does not. The Arkenfox user.js does all of this and more, but these options are not enabled by default. This shows that Firefox does not care much about privacy in practice.
The only "breakage" that I have encountered with such a hardened configuration is related to the spoofing of the time zone. But the fundamental issue is that Javascript/browsers should have not been designed to allow websites to extract this kind of personal information in the first place. But even that is not enough and users are still fingerprintable. In an ideal world, the only thing a website should see is the originating IP and nothing else.
If anything, Brave has done more to harden Chromium than Mozilla has with Firefox, even though Brave comes with its own set of problems (scammy crypto integrations, AI, VPN and other stuff).
