I read that disabling IPv6 speeds up browsing and is recommended from a security perspective. Is this true? IF I disable it, how many sites will I not be able to access? According the Mozillazine, Firefox first tries IPv6 and if that fails, tries IPv4. Will disabling IPv6 reverse the preference, or disable it entirely?
I'd not bother. Most websites arn't IPV6 ready, but whether you look for IPV6 is a system level thing, not a browser level thing.
In general, turning off IPV6 hasn't really shown a subjective difference in performance, and most systems don't have it anyway. It won't hurt, but it won't speed anything up.
Trying IPV6 first and IPV4 next is the current behaviour in a dual stack system (that is to say one that has IPV6 natively, a tunnel, or miredo set up). I'd leave it as such.
Securitywise, having working IPV4 its no different from having a public IPV4 IP, and well, NAT isn't security. Just exercise common sense and not run garbage you don't need, and have a good sensible firewall set up, and you'd be safe.
I ran an IPV6 capable server a few years and apparently most brute force attacks ignore it cause of the massive address space.
Actually the web browsers implement something called Happy Eyeballs. Which means the browser check both IPv4 and IPv6 at the same time, and uses the connection which succeeds first. It is design to minimize the delay experienced by the user.