The parsing of URLs could have changed.

Does the actual code have the protocol and host name in the URL, e.g.,

newWin.document.location = "http://www.example.com/ReportViewer.aspx?etc.";

No, http://www.example.com is not part of the URL. The URL is being sent from the window that opened this one. The other browsers and the previous FF versions seemed to understand how to deal with that. I tried adding the protocol and host to the URL and now version 31 responds correctly. I guess I’ll have to modify my code to account for your improvement(?), or suggest my customers use another browser.

Your post was sort of helpful since I can correct the problem with FF's changes but I wonder how many other WEB sites were also broken.

I tried to search for the source of this change last week but didn't find it in the bug tracking system. I'm assuming it was intentional, but if the pattern of using a relative URL is widespread, then you or someone else affected by the change, could file a new bug to address it.