Due to the Dutch certificate authority Diginotar being compromised, Mozilla released Firefox 6.0.2 and Microsoft released security advisory 2607712 in order to prevent those compromised SSL certificates–in layperson terms, a file that permits web servers to use https for security–from being used.
Without this step, someone could use a compromised certificate to set up a fake web site masquerading as some other web site you trust and using it for fraud.
Needless to say, it would be a very good idea to update Firefox–from within Firefox, click Help, then Check for Updates–and if you don’t know if Windows has updated itself in the last day or two, visit http://support.microsoft.com/
As far as I know, Mac users who use Safari have to use Keychain to remove the Diginotar root certificate in order to protect themselves. Since I don’t own a Macintosh, I can’t tell you any more than that. I can only repeat what I’ve read. If you run Firefox on your Mac, updating to version 6.0.2 should protect you.