The problem is that the browser owned control bar is right next to the web content. It's good for usability (quick to reach with mouse), but opens the door for trickeries like click jacking (that's actually why there's a countdown when you want to install an extension in firefox).

I filed a related bug a year ago at Mozilla: https://bugzilla.mozilla.org/show_bug.cgi?id=497388 but it didn't get anyone busy (no offense). We probably have to wait until browser extensions are more common place and these kind of UI vulnerabilities get exploited on a regular basis.