Win98 you know... when something does not work you can:
1 - Reinstall win98 and try it again;
2 - Stop trying to install the program you are trying to install...
I've used Win98 and SE for a long time. Some games, specially Eletronic Arts could randomly give the very same problem you are having with Mozilla. Even deleting the game and reinstalling it again, the problem would persist. The solution is to format and install the game again. Try using a descent Windows (NT based systems rocks) or Linux. Win98 and 95 are very buggy. If you use it "as is" it works fine. When you start installing stuff, upgrading, beware, it will give you headache