If you use the add-on IE Tab or one of its derivatives, can you check whether there is an option that controls this? ---- If you just left-click or if you Ctrl+click (open in new tab), do those work normally or do those also launch an IE window? ---- The .aspx extension can be used to delivery ordinary web pages, PDFs, or any other kind of file. Firefox generally looks at the content-type sent by the server. Firefox compares the content-type with the list of application handlers to determine how to open it. The content-type is invisible in ordinary browsing. You can view it using an add-on, or perhaps more quickly if you use Rex Swain's HTTP viewer tool. After you a logged in to Ancestry, paste the URL into the form on [http://www.rexswain.com/httpview.html] and click the bubble for HEAD then click Submit. On the results page, if you scroll down a bit to the "Receiving Header" section, what Content-Type is listed there? With that information, we may be able to find a problem entry here: orange Firefox button ''or'' classic Tools menu > Options > Applications