Firefox 3.6 hangs completely when attempting to print. Reboot of win98 is necessary because the rest of the system (mainly explorer) is not very usable. But it does not hang the whole system.
Now, in KernelEx 4.5RC4 things get a bit better. Print preview works fine. I can even press print and choose the printer. But it bombs out when klicking Properties (of the printer). The printer is HP PSC 1510. Windows shows a blue screen of death. Accepting it, it comes back but Firefox is closed. Running it again works, but this time print is dead. Print preview works, but klicking print hangs Firefox as in the original report. It is possible to kill it from the Ctrl-Alt-Del menu. Running e.g. Excel doesn't work now and Windows Explorer hangs soon too. It is possible to shutdown the machine from the Ctrl-Alt-Del menu.
I should add that I do not have kernelex enabled globally. Only the firefox executable is allowed to run in Win XP mode. Maybe it is then trying to use some WinXP print APIs which are not available, because the print spooler is not in Win XP mode?