I happen to love older versions of Mac OS X, but one place where they fall short nowadays is HTTPS support. On OS X 10.9, the Dictionary app is no longer able to connect to Wikipedia, and Apple Mail is filled with broken remote images. To fix these problems, you need a proxy server, and specifically a "man in the middle" proxy which can translate between modern HTTPS protocols and older TLS implementations.
The Squid proxy is a lightweight, open source, battle-tested, and Mac-compatible option. It's a bit tricky to set up, but I've created a pre-configured installer to streamline the process, which will work on any Mac running OS X 10.6 or newer. If you're using a version of OS X that's more than seven years old, you should definitely give it a try!