Firefox isn't going to gain any valuable publicity that they don't already have by announcing "Firefox X is here". Unlike Chrome, they're not in a position to be making rapid gains - a large portion of what's left of the market is companies using older versions of IE for compatibility reasons, which aren't going to switch over on a whim.

Chrome had an advantage - it took a bite out of the large firefox market, which was already prepped for 'trying new browsers'. While the constant 'Chrome Y released!' helped, Chrome would probably have risen nearly as fast without it.