It depends on which level you want to see the source code.
Firefox's View Source gives you the source code as seen by the render engine; unfortunately, FF itself applies a few transformations before the render engine gets to see it, and there's some caching involved, so it's not always reliable.
Tools like curl and wget will show you the exact response at the HTTP level (including HTTP headers, if you ask for them). This is usually good enough, and they'll transparently handle HTTPS for you so you don't have to worry about the SSL part of the protocol.
If you want to dive a bit deeper, try netcat or telnet - these basically give you a byte-stream connection to the server, allowing you to type (or pipe) HTTP requests directly, and view the raw response. The downside is that if you violate the protocol, you'll be kicked out of the connection.
To go yet a level deeper, consider a network sniffer - wireshark is excellent, but fairly complex; it will give you the raw packets as well as their contents at different levels of the network stack.