On gecko engine your best options are mullvad, librewolf, and tor browser. Dont use firefox as the other options are categorically better.
Although firefox is still usefull if you want to present a unique fingerprint to certain websites to throw them off your track. (Keep them on a dedicated fingerprint).
Also know that many of the security objections thrown at the gecko engine based browser are actually mitigated if you set the browser to erase all data on exit, and by not loading multiple websites at the same time, if site isolation is your concern.
Mullvad and librewolf are considerably better at anti-fingerprinting than chrome based options. Though you gotta use them with a vpn to benefit from that.
Ungoogled chromium is google free but has no protection against fingerprinting, so it really doesn't help your privacy much, and probably makes you quite unique. Combined with its update difficulty, makes it a bit un-useful.
I would recommend you change your mind on brave as despite their corporate nature they are doing an applaudable job of presenting a somewhat private chrome based browser. The only one available in fact.
Remember that there just aren't that many good options out there and if you want at least "some" privacy then your choices are essentially the four I mentioned.
Also my last read about librewolf, it is extremely close to mullvad with minor differences and not haphazardly put together. Also mullvad is identical to tor browser, minus the tor network.
Check this out:
https://privacytests.org/