Hello parthasarathi, '''you must install a security software''' because firefox(and all others browsers) does not have one. see a great and useful add-on also : [https://addons.mozilla.org/en-us/firefox/addon/noscript/?src=hp-dl-mostpopular NoScript add-on] thank you

Firefox does give some phishing and attack site warnings *http://www.mozilla.org/en-US/firefox/phishing-protection/ Firefox does work with software that you may have installed and for instance may enable scanning of downloaded files. If it is scanning now that indicates you have something installed. I believe Windows 7 makes Microsoft Essentials available for free. Windows 8 has Windows Defender or something provided by default. * http://windows.microsoft.com/en-GB/windows/security-essentials-download The no script add-on is an extremely popular add-on that blocks a lot of nuisances and potential dangerous things. Firefox may soon include a click to play plugins feature that should be helpful. * https://blog.mozilla.org/addons/2012/10/11/click-to-play-coming-firefox-17/ BTW are you using Thunderbird ?