[willemlabu]

Adding an 'f' in front of the word 'mining', just to make all the words in the list alliterate. :) It's pretty funny to me, and shows the author is just ever so slightly making fun of the compulsiveness and repetitiveness in the game.

[rkuykendall-com]

I also thought it was a reference to the "Four Fs" of evolutionary psychology where the final F is often switched out depending on the environment: Fighting, Fleeing, Feeding and... mating.

https://en.wikipedia.org/wiki/Four_Fs_(evolution)