One thing: I reread your post more carefully and should note: the main site (www) does
not support SSL at this time.
However the webserver software gets a little confused if you try to access the site through HTTPS anyway. The webserver is listening on 443 (HTTPS) for the sites (forum & server hostname) that need it. I guess if you then request any other site (that technically is only configured for non-HTTPS port 80) it will serve that site but will just pick the first SSL certificate in its config to serve with it. Hence why you get the certificate error.
I do not want to make the site (www) HTTPS at this time as it technically means breaking all old links that still link to the HTTP (non-SSL) site. As users will be redirected they might not notice it really, but search engines will. However I may have to get an SSL certificate for the site anyway if only just to redirect any traffic that might stumble on it to the "proper" site (HTTP). A bit weird, but oh well.
Also for Firefox not to support SNI it would have to be ancient. SNI is supported since Firefox 2.0 (released in 2007).
And yeah, fuck IE, or at least on XP: no version of Internet Explorer on Windows XP supports SNI. You need at least Vista for SNI support in Internet Explorer. Other browsers (Firefox, Chrome) on XP are not affected by this (assuming they're somewhat recent versions). I have no problem with the forum not even loading in IE6 (or any IE on XP) anymore: people should stop using it anyway.
As
Let's Encrypt will be nearly ready (free SSL certificates for everyone) and Firefox will probably mark login forms as insecure if not served via HTTPS from early 2016, you are probably going to see a lot more people use SSL and good chunk will also use SNI.
So to recap:
https://galactica.visei.net/ should work without errors or warnings (but will redirect you to
http://www.emulator-zone.com)
https://forums.emulator-zone.com/ should work without errors or warnings
https://www.emulator-zone.com/ will
not work and give errors -- use
http://www.emulator-zone.com instead