Some attack sites are displaying this dialog to induce you to install unwanted software, with distribution through ads or affiliated sites. It also could be caused by an extension installed in your Firefox (your extensions weren't list in your More System Details, so I don't have specific thoughts on whether any are unsafe). You can review/disable/remove extensions on the Add-ons page. Either: * Ctrl+Shift+a * orange Firefox button (or Tools menu) > Add-ons In the left column, click Extensions. Then, if in doubt, disable. You might also want to run some supplemental malware scans. This article lists tools other Firefox users have found helpful: [[Troubleshoot Firefox issues caused by malware]].