Many sophisticated security suites monitor for program changes and distrust modified versions of program files. But this approach doesn't easily distinguish between changes caused by an update vs. a virus. Usually there is a way to instruct your security software to trust Firefox again. Perhaps try the steps in this article: [[Fix problems connecting to websites after updating Firefox]]. Any luck?

Also, the problem could be related to a feature of Kaspersky that filters your web requests for potential bad stuff. In order to filter secure connections (HTTPS URLs), the security software presents a fake certificate to Firefox so it can intercept and stand in the middle of the secure connection. To have Firefox trust these certificates, you may need to do something such as import a root certificate, or click something in your security software's settings. Here are links to some documentation on this SSL Scan feature: (1) General SSL scan setting: [http://support.kaspersky.com/6688 How to scan encrypted connections in Kaspersky Anti-Virus 2012?]; [http://support.kaspersky.com/7935 How to scan encrypted connections in Kaspersky PURE 2.0?] (except: turn it off instead of on) (2) Making an exception for a specific program (e.g., Google Chrome): [http://support.kaspersky.com/7470 Encrypted connections scan by Kaspersky Internet Security 2012 in Google Chrome]; [http://support.kaspersky.com/9061#block0 Scan of encrypted connections in Google Chrome and Opera (KIS 2013)]