Another update...
Used ISO Buster 1.7 to convert the DMG to BIN/CUE. Some DMG files successfully converted but some didn't. I guess the failed DMGs are Apple's new format of DMG. Unluckily Firefox uses this new format I think.
Further tests...
Out of those DMG files which got successfully converted, bootable software CDs had read errors in the iMac. Software CDs without boot information were properly readable in iMac.