"Your Firebug show is never coming back."
-Strong Bad

As much as I love Firefly, I would not press the button. Firefly had its chance, and it didn't last long. Most of the fans (myself included) only became fans after it was cancelled. It has almost a kind of mysticism about it because of that. Bring it back and it'll lose that vaunted "what if" that makes it such a precious commodity. While Firefly is certainly a great show, the "what could have" is often better than the what is and doubtless contributes to Firefly's mystique. Sherlock, on the other hand, has no trouble pulling in the millions when it's on. I personally wouldn't sacrifice the surety of Sherlock in the hopes that Firefly can become a phoenix.

That said, I would totally watch new Firefly should it ever happen.