"Update, 12:30 p.m. ET: Several security groups are reporting that it is extremely easy to get whacked by this vulnerability/exploit just by visiting one of a growing number of malicious Web sites that are now employing this attack. F-Secure's blog post on this indicates that -- because the vulnerability lies in the way Windows parses WMF image files -- Firefox and Opera users also can get infected -- although they at least have to agree to download and run a file first"

That's what they say in the article but the only thing I did was to open a .wmf movie in Firefox. I did not click/agree/install anything else.
The thing just auto-installed it-self from that point.