If they use, say, Mozilla or Firebird/thunderbird, they will be immune to email viruses.

Evidently a Mozilla virus has been found.

From the danish daily "Jyllands Posten" today (translation follows):
Derfor har mange overvejet at skifte til andre browsertyper, men fejlen i Mozilla er lige så alvorlig som sårbarheden i Internet Explorer. Sikkerhedsmæssigt er der derfor ikke noget formål i at skifte browsertype.

Thus many have considered to change to other browsertypes, but the fault in Mozilla is as serious as the vulnerability of Internet Explorer. Safetywise there is no purpose in changing browsertype.

Erik