I reported a strange behaviour a week ago, where some pop-ups were not being blocked correctly.
I have now done some further investigation, and it turns out that this only happens on system where Macromedia Flash Player has NOT been installed. And, furthermore, it is the use of Flash in certain banner ads that makes a couple of new windows pop up on these systems.
You can re-create this by visiting the URL below - keep hitting until the HP Invent banner is served. After that you should have two new Firebird instances. Tested on Windows only.
http://adserver.adtech.de/?adiframe|2.0 ... et=_blank;