I am using Mac OS 6. The problem began with recent upgrades of FF. A PDF file that a website provides as a what seems to be a "pop-up" (i.e. document appears without initial appearance of an icon in the either the "download" folder or the set of icons in the lower blue bar) does not appear. If I go to the SAME website using Safari, the PDF file appears as expected. Perhaps FF has a pop-up filter that I can adjust. I just experienced the problem when attempting to download my monthly checking account statement from my bank's website (i.e., not successful with FF; successful with Safari). The problem ONLY began after upgrade of FF - perhaps to v.8.0. I cannot provide an example URL because I experience the problem when using web sites that require a log-in.