Firefox's warnings are based on Google's blacklist. Recently, a bug was reported in Firefox 17 that causes sites that were previously blacklisted to remain marked as "Reported attack sites" even after they were removed from Google's blacklist. You can read the bug report.
The Firefox team is aware of the issue, and they have fixed it in the new version of Firefox. Firefox 18 is scheduled to be released to the public tomorrow, 8 January 2013, and thanks to the automatic update utility in Firefox, many users don't even have to manually update their browsers in order to get the newest version. So this issue should be resolved as of tomorrow; if you still receive warnings, you should try updating Firefox manually to version 18 if it hasn't updated automatically.