I think the last PowerPC Mac that Apple was shipping was the PowerMac in 2006 (PowerMac which was replaced by the Mac Pro in June 2006).
From the data I've seen most of the web fairing Mac users are running Intel. Mac users are very fast to upgrade not only there operating systems but also to replace there old hardware.
Personally I've had 2 Intel Macs (MacBook Pros) already since 2005 when I had a PowerPC based PowerBook. So I'm not really that bothered about the news. If this helps Mozilla focus and improve the software it's all good by me as I do use Firefox on my Mac.