I'm using Mozilla Firefox on a Mac OS 10.6.8 operating system, although I don't think that information is likely to be relevant as up until today I've been using this setup and nothing has gone wrong.
When editing pages in the Advanced Editor, instead of opening pages in new windows the page loads in the same tab and window as the editor. When I click "Close and Save Changes" it directs me to a page saying "you may close this window". I haven't recently changed any of the settings on my computer or on Mozilla Firefox. I'm going to have a look around now but if anybody has had a similar problem before (or is having the problem now) and knows how to fix it, please let me know. Otherwise it might be a site bug.