Here is Firefox's page about attack sites and malware.
Malware is software designed to infect your computer without your knowledge. Malware is most often used to steal personal information, send junk e-mail (spam), or spread more malware.
Attack Sites are Web sites that try to infect your computer with malware when you visit. These attacks can be very difficult to detect; even a site that looks safe may be secretly trying to attack you. Sometimes the Web site’s owner won’t even know that the site has been turned into an Attack Site.
You can learn more about malware and Attack Sites from StopBadware, a non-profit organization that works with partners like Mozilla to protect users from malware and other dangerous software.
From StopBadware, there is a way to ask them to remove your site from the list after you have removed the malware.
Help! This is my site.
We know this is frustrating. To remove the warnings, you'll need to find the problem, clean it up, and request a review. We can help you do this.
StopBadware uses blacklists from various sources. After you have removed the malware/hack from your site, you will have to use their Clearinghouse Search function to figure out which blacklist you are on. Then you will have to request a review from that blacklist organization.