Hi AB, could you describe what happens when you try to view a PDF? Is there a popup warning message, or some other sign of malware? Meanwhile, could you test in Firefox's Troubleshoot Mode? In that mode, Firefox temporarily deactivates extensions, hardware acceleration, any userChrome.css/userContent.css files, and some other advanced features to help you assess whether these are causing the problem. ''If Firefox is running:'' You can restart Firefox in Troubleshoot Mode using either: * "3-bar" menu button > Help > Troubleshoot Mode... * (menu bar) Help menu > Troubleshoot Mode... and OK the restart. A small dialog should appear. Click the Open button. ''If Firefox is not running:'' Hold down the Shift key when starting Firefox. (On Mac, hold down the option/alt key instead of the Shift key.) A small dialog should appear. Click the Open button. ''Note: Don't use the Refresh without first reviewing this article to understand what will be deleted:'' [[Refresh Firefox - reset add-ons and settings]]. Any improvement? Ref. [[Diagnose Firefox issues using Troubleshoot Mode]]