Your "More System Details" didn't include information on your extensions, but one of those could be responsible. Could you check for unrecognized 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 that something is valuable and useful, disable it. Usually a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step. Does that change how Firefox behaves? ---- You might also want to run some supplemental malware scans in case your regular security software is missing something. This article lists tools that other Firefox users have found helpful: [[Troubleshoot Firefox issues caused by malware]].