User-Agent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X; en-US; rv:1.8.1.13) Gecko/20080311 Firefox/2.0.0.13 Build Identifier: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.5; en-US; rv:1.9b4) Gecko/2008030317 Firefox/3.0b4 Adding a vbox to the Firefox DOM using an overlay will show the window over the chrome part of the window, but not the browser pane. This behavior affects Firefox 3 only, not Firefox 1 or 2. Reproducible: Always Steps to Reproduce: Create an extension with the following overlay to browser.xul: <?xml version="1.0"?> <overlay xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul" <window id="main-window"> <vbox style="z-index: 100; background:red; top: 0px; left: 90px; position: fixed; height: 400px; width: 400px;"/> </window> </overlay> Actual Results: Red box sits on top of the chrome (meaning the toolbars), but not the browser pane. Expected Results: Red box sits on top of the chrome and the browser pane. Reproducible on at least MacOSX and Windows