Go to Tools | Options | Advanced | Update (tab) and checkmark "'''Ask Me What To Do'''" in the centre menu. But sometimes it's in your own interests. For example, v3.6.16 which you appear to have installed is a vulnerable to exploits. There was a security breach at Comodo which is an SSL certificate provider recently whereby a number of fraudulent certificates were inadvertently issued. These allow a hacker to impersonate any site including online banking and the Firefox version you're running at the moment will not warn you that the site is a fake. The fraudulent certificates were blacklisted in v3.6.17 and beyond. For more info, see http://blogs.comodo.com/it-security/data-security/the-recent-ra-compromise/