The spam appears as a message telling the recipient that their website is incompatible with Mozilla-based browsers and asks the reader to fix the "problem".
"The email arrives with an attachment, supposedly a screenshot, so that the recipient can examine the problem," said Roel Schouwenberg, senior research engineer at Russian anti-virus firm Kaspersky on its weblog. "Of course the attachment isn't a screenshot at all - it's an .exe file."
The ruse is making use of the fact that some websites will not operate under certain browsers and appears to be an attempt to fool the net-savvy who traditionally suffer less at the hands of spam and viruses.
2005 has been a good year for Mozilla. In March SC revealed details of a study that showed Mozilla's Firefox was free of known vulnerabilities for 85 percent of last year, as opposed to Microsoft's Internet Explorer which was only safe for two percent.