Win 7 and XP, FF4.0.1 Test the page: http://www.peakoil.org.au/news/index.php If I click on one of the titles in the navigation iframe, I get the article in a new page, not in the "article" iframe. This doesn't happen in FF 3.6.17 , and it doesn't happen in IE8. I disabled all the add-ons in one go and it started working OK. I re-enabled all the add-ons one by one, and it kept working ! The relevant HTML/PHP on the server won't display properly here, but can be seen at http://www.peakoil.org.au/news/html-php.txt