When Build 18 of Mozilla tries to launch, an error with GKHTML.DLL appears after the "Mozilla" splash screen comes up on the screen, and the program closes.
Try a newer nightly build. If that works, it is most probably a bug in M18 which has now been fixed.
please try a new nightly. Also, if oyu have ns6, this will cause the outdated m18 to not work. be also sure to remove all mozilla files before installing a new build.