Try to run xulrunner directly from /Applications/Eclipse_3.6.1/XUL_1.9.1.2:
$ cd /Applications/Eclipse_3.6.1/XUL_1.9.1.2
$ ./xulrunner -version
does it works for you? what does it say?
I'm not sure what is in universal xulrunner package, probably it works only for 32 bit intel/ppc, but G5 is 64 bit. I downloaded xulrunner sdk 1.9.1.2 from http://mirrors.directorymix.com/mozilla/xulrunner/releases/1.9.1.2/sdk/ for i386 and powerpc. Archives have xulrunner binaries in xulrunner-sdk/bin folder inside archive and they are different.