TestOpenInTargetFrame attempts to click on a link <a href="./frame_to_be_loaded.html" target="leftFrame" id="show">. In IE, we just call element.click() and this just works, but in Firefox we have to simulate the effect of clicking on the link, by changing the location of the target window to the specified href. In non-PI mode, we keep a JS list of all window target names, but in PI mode, the JS can't know which target to update, so you get the error: "Window does not exist" when the link's target can't be found.