It seems that 10.4Fx has come out just in time, as Josh Aas from Mozilla reports that the "universal" builds of Firefox 4 beta 7 are now i386/x86_64, not the traditional definition, which is of course i386/ppc.
So this is where the new era begins. Welcome to TenFourFox, where PowerPC lives on. 10.4Fx doesn't support all the features of Fx 4, and may not ever (particularly features like out-of-process-plugins may even be irrelevant on PPC, given that Adobe Flash will be x86-alone any day now). But it does support those of us who still have perfectly good Power Macs, including those of us who still use them for Classic applications, and it includes all the great layout, HTML5 and CSS3 enhancements that are part of Gecko 2.
This blog covers some of the development progress so that you can stay informed. I'll probably be working up a Twitter account next, but new developments will get posted here first.