The download page says gecko 18.104.22.168, but when I open my old installation the useragent shown in Menu>Help is:
Mozilla/5.0 (Windows; U; Win98; en-US; rv:22.214.171.124) Gecko/20090825 K-Meleon/1.5.3
Could well be that the gecko engine inside was updated some time later, that was usual at the time.
According to this list gecko 1.8.1 is Firefox 2 (!!)
But why in the world do you want to use such a horribly obsolete version?? I'm not usually someone nagging others to update everything
but even on win98 KM1.6 (gecko FF3.5) can be used! Okay, only after some OS updates and KernelEx, that's a bit tricky. But at the very least would try KM1.5.4, that still runs natively on w98 and contains some bugfixes and other updates.