I don't have an account to test with, so I can't pinpoint the cause of the problem. Generally speaking, in Firefox, website scripts have permission to close a window if that window was opened using a script. As far as I know, this has not changed at all in Firefox 6. However, it is possible that the USPS script is crashing on a different step before it closes the window due to changes in Firefox 6 (or changes in the USPS script). When did the site stop working correctly (i.e., after which upgrade)? My first suggestion is to report this to the USPS webmaster if you find an easy way to do that. If not, post back and someone here might be able to find a way to do it. Second, until this is solved, you might want to use Ctrl+w to close that window if your hands are on the keyboard anyway.