This has to have already been reported, but I couldn't find any existing similar bug... If I am viewing a web page with a Java applet (chat window, radar loop, etc.) in one tab and click to go to another tab, the Java content from the first tab is shown over the content in the second tab. I'm seeing this on Firefox 1.0 but the bug has been present on all Firefox versions I've tested (back to 0.6). I don't remember seeing this on the Application Suite (Seamonkey).

For "it must have been reported" the "Find a specific bug" form with "all" rather than "open" generally works quite well, though in this case the "Most Frequently Reported Bugs" page also works: number six at the moment. *** This bug has been marked as a duplicate of 162134 ***