It actually was pretty easy. All I did was install Fennec and then tweak a few settings.
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I switched the browsing protection to strict
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I installed ublock origin
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I went to about:config and enabled resist fingerprinting
I ran some fingerprinting tests and my slightly modified Fennec performs about the same as Mull
I spent the whole day figuring this out, i did the same as you with fennec and hardening. Took some inspiration from arkenfox and betterfox and thats about it.
Edit: its sad, but such is life i guess. I loved mull.
Mull you will be missed