Favicon isn't changing for me in Firefox 33. Also worth noting that it still triggers when it's the frontmost tab in an unfocused window. I'd pulled up a smaller window on top of it, so the change to Gmail was hard to miss.

Despite that, I can imagine people falling for this even if the favicon doesn't match. They'll just say "Oh, Gmail's favicon is wrong. Silly web browser." If they notice the favicon at all.