Had weird issue recently: TinyMCE couldn't open popup boxes in Firefox for same origin policy violation.
Solution from TinyMCE Wiki wasn't of much help - being that dozens of web spaces are accessing same JS instance. After hour or so browsing the web for a hint, i stumbled upon a forum post (can't find it right now) where author asked for help setting up apache proxy to fix same origin policy violation. And that's what fixes it:
ProxyPass /core http://core.cmass.info/ ProxyPassReverse /core http://core.cmass.info/
Now, you can access core files as if they were on same domain, e.g. http://www.massvision.net/core. Busted!