The download page says gecko 1.8.1.21, but when I open my old installation the useragent shown in Menu>Help is:
Mozilla/5.0 (Windows; U; Win98; en-US; rv:1.8.1.23) 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 (!!)
https://developer.mozilla.org/en-US/docs/Mozilla/Gecko/Versions
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 :cool: 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.