There is a browser called "Roccat" which supports OS X 10.5 or later, PowerPC or Intel, but it still uses WebKit, so unless you have Leopard-WebKit installed, it is just as insecure as Safari.
Actually, whenever I get around to buying a suitable PowerPC(which I have been planning on since 2015, but never got around to it), I *might* be able to start compiling "Stainless", it has been discontinued by its developer, and it's(OS X) source code is up on github.
Speaking of compiling, I wonder, if Darwin is open source, would it be possible to compile a newer kernel for PowerPC's?