Safari has been patched against the carpet bomb vulnerability with a new version of the browser: http://www.apple.com/safari/download/
However it is either use Safari or Fx, because the users that have both Safari and Firefox on their boxes are still vulnerable in Firefox: http://xs-sniper.com/blog/2008/06/20/bk-on-safari-hunting-firefox/
So a combination of different software on one computer can be a added security risk,
So many different combinations of Applications, there will never be a perfectly secure system. But Apple is working closer with MS lately, as you can see by Exchange support being integrated into OS 10.6 and the iPhone. So hopefully this means they are working together on security issues as well.