Firefox does not have traditional antivirus protection. It does include some site and URL reputation checking features, but those cannot protect you from what actually gets through to your system. Of course, there are many brands of security software, some of which are free for personal use. You can find reviews on reputable sites such as the following: * PC Magazine: http://www.pcmag.com/article2/0,2817,2369749,00.asp * Computerworld: http://www.computerworld.com/article/2943119/security/security-suites-choosing-the-best-one-for-you.html * AV-Test: https://www.av-test.org/en/news/news-single-view/internet-security-packages-the-best-products-at-modest-prices/ You also could consider using OpenDNS, which is free for home use. It checks each request and blocks untrustworthy sites and downloads. https://www.opendns.com/home-internet-security/

I should add that if you discover Firefox can't access any secure (HTTPS) sites after installing or updating security software, then Firefox may need to be set up to work with a feature included in some packages to intercept and filter your web browsing. In other words, don't panic. ;-)