Steps: 1. Make sure you don't have adblock installed -- that cures the bug 2. Use on a machine 2. Load test case 3. Make sure the browser window is large enough to so no scrollbars are necessary 4. Reload page with ctrl+R Plugins disappear What's happening is that nsObjectFrame::Reflow() happens before it knows that it is a broken plugin. For some reason the bug does not happen on initial loading of the page, or when going back in history to the page.