When I am on ancestry.com and researching various genealogy family members. If I right click an ancestor and do "open in new window," instead of opening in Firefox 12.0 (which is set as my default browser) it opens in IE. If I click on the properties of the item(s) I'm opening to see what they are, they say they are ASPX files. Obviously the URL can be opened in Firefox after it's been opened in IE, but not directly with a right-click from Ancestry.com. Why is that? (Note, you may need to be a ancestry.com member to see the info displayed within this URL.)
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