Mull works the same as Fennec, except it is hardended with patches from Tor and Arkenfox user.js. No real reason IMO to use fennec over Mull, whose developers also contribute to Fennec. Ghostery also changes your fingerprint, acting as one more data point. Mull has a whole bunch of configured flags to reduce fingerprinting, and many more to help with security (like disabling JIT).