I think it is far be it from me. Actually I hate when it's misspoken like this. *is sad*
It's spawned its own term among the re-enactor community: farby. Farby, or farb, is used as an adjective: "We don't do events with them. They're a farby unit," or "John, what are you doing with that farb hat?!" It comes from "Far be it from me to criticize your _____" (e.g. uniform, drill, manual of arms, etc.) "but..." Then they go on to criticize.
It's not for me (!) to question, but why are we arguing about two different phrases that both have validity? The one listed here has useage. Mia', tell the dissenters to put their own word up and argue over there. My life experience is with your version. First I've heard of theirs but I'd be happy not to start using it. Far be we from the Verbal Arms? For the Arms, anyone?