Both are good. Librewolf is more like vanilla Firefox, just configured way better by default. Mullvad Browser is like a port of the Tor Browser (also based on Firefox) for the clear web (or for use with Mullvad's VPN, or whatever). Also configured very well by default. Mullvad Browser has better anti fingerprinting stuff built-in but as a result of its unusual configuration some sites might be broken. Librewolf is kind of the opposite in that regard - sites won't be broken but you'll be easier to fingerprint. In any case, they both are at the top of the best Firefox variants I'd say.