So I transferred the newest version of TenFourFox (45.6) to the Pismo G3 laptop's startup drive which is a CF card in a FireWire 400 reader by connecting the CF card to the MBA via Thunderbolt-FW 800/400 adaptors. Then reconnected it to the Pismo and booted up. It didn't do so, just sat there with a pale grey screen & dark grey Apple logo & spinning gear wheel.
So I reconnected it to the MBA & used DiskWarrior 5 to fix the CF card's directory.
That did the trick, it then proceeded to boot normally.