It's a custom build of Firefox with somewhat sensible, sometimes strict, privacy respecting default settings.
There's also the Arkenfox user.js which you can put on top of vanilla Firefox, aiming for the most privacy and security possible.
https://github.com/arkenfox/user.js
My issue with these browsers, including Firefox with things like fingerprint resisting enabled, is that it breaks a lot of sites. Add a VPN to the mix and a lot of sites flat out refuse to let you interact with them, or they give you 5 minutes of captchas, or they require 2 factor login despite asking them to remember your device. I have to open some sites (banking, brokerage, health insurance) on a near-daily basis in Chrome with no extensions and no VPN instead of my regular firefox+vpn.
A lot of sites allow interaction even with the above but they shadowban you without telling you. Craigslist shadow bans and auto-spam-filters any submissions done with a VPN, and then also auto-spam-filters any subsequent submissions on the same account even with the VPN turned off.
Reddit also universally spam-filters any submissions and comments done under a VPN, and rate limits your commenting a shitload on VPNs.
Arkenfox is great, although worth noting that there are always privacy vs. security vs. usability tradeoffs. The best usability settings (in terms of sites just working at least) are generally the Firefox default and Arkenfox defaults aims for privacy mostly but they also have some of the best descriptions of available configuration available anywhere (often the only other source of any kind of information is a brief comment in the source code that assumes familiarity with Firefox code). Personally, I aim for the best security and accept that that makes me unique.
It's a custom build of Firefox with somewhat sensible, sometimes strict, privacy respecting default settings.
There's also the Arkenfox user.js which you can put on top of vanilla Firefox, aiming for the most privacy and security possible. https://github.com/arkenfox/user.js