Try installing AVG 2013 free - integrates with browsers and provides a little bit more protection.
FF is becoming bloated, latest release is quite odd at times, now refuses to have to do with my my webmail (normally use Outlook but sometimes use a machine that is not configured for it).
Chrome is ok, but Google is now integrating so many of its services into one package that you have to be careful if you start to use anything other than the browser bit.