From marketing point of view it's not a wise move. Consider how much press announcing 'Firefox X is here' they lose. And each of these announcements can potentially attract new users.
I think Chrome's fast growth can in part be attributed to by the constant stream of news like 'Chrome 1,2,3,... released' which helped Google to build recognition.
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.
I think Chrome's fast growth can in part be attributed to by the constant stream of news like 'Chrome 1,2,3,... released' which helped Google to build recognition.