if you are viewing a page and type Ctrl+N you get a new window which then loads the same page you're viewing.

Hmm, there is a duplicate tab option that you get when installing the tab browsing extensions. You can try the clone window extension, it looks like what you want but I have never personally used it: http://www.pikey.me.uk/mozilla/?extension=cw

Dunno why you are having problems with that theme page site, I use it all the time and have never had that error. They have the best extensions area too IMHO, if you can get in: http://www.texturizer.net/firefox/extensions/

You could try the official mozilla theme and extension site, but it really does kind of suck: http://update.mozilla.org/