Hi,
Note that Firefox (or any other browser) is a reader, so is very unlikely that only by surfing the Web you got any kind of Virus. You need to have installed something on Firefox to get that kind of report.
The only place in Firefox where you install things is through ADD-ONs, where in this case is the
DealPly (dealply.com) Add-on that is messing around. Uninstall it from Firefox and you get the problem solved.
Regards,
Rui Seixas Monteiro