My MacBook Pro 2012 running ML has started not to load certain web pages. I tried with every browser but I have the same issue. I thought it was a firewall trouble, but it's disabled. Tried to ping websites, dns resolving works. Can't load even from IP addresses. Also apps that needs some internet contents can't work. I can just surf with tor p2p network.
I used to get this mainly on banking sites. It might be a certificate issue, or more accurately, the older browsers that run on ML not being compatible with the certs or negotiate secure connections properly with the sites (perhaps the server actively denies access due to browser version).
As ping and DNS works, the problem must lie at a higher level.
Unless you've a reason to run ML still (I've only just updated to ElCap due to various bits of software) then I'd update.