Mullvad browser
I don't mean this as a dig or anything but it's literally what they described.
Yes, which is not Firefox with default settings
For the average user there's not really a particularly good reason to jump for a downstream browser unless you are specifically using their main features (tor or VPN). It's easy enough to get 95% of the way there with 30 seconds of config tweaks off stock upstream.
The extreme bent towards privacy and not retaining any identifiable or fingerprint-able behaviors is exactly what its target audience wants, of course. But it leads to a markedly more inconvenient experience than Chrome or default Firefox or Safari.