Scammers are trying to con gullible marks into getting infected via a fake Firefox 4.0 beta download scam.
Preview editions of the next version of Firefox are available for download directly from Mozilla at no charge (natch). So it's pretty obvious that offers of Firefox 4 beta cracks or a keygen are entirely bogus and almost certainly malicious.
Fraudsters are therefore hoping the hoodwink the truly clueless into running a supposed Firefox 4 keygen tool or crack, both of which are being promoted via scam-boosting Twitter accounts.
The crack and keygen are both infected with a Trojan downloader, warns Sunbelt Software.
Surfers who follow through with the scam are also directed towards a site hosting a smorgasbord of other digital parasites, as explained in a blog entry by Sunbelt researcher Tom Kelchner here. ®