Try running Firefox in [[Safe Mode]]. If it functions properly in that configuration, then one of your add-ons is the culprit. Firefox Safe Mode does not disable your plugins, only the add-ons you've added yourself, or have been added by something you've installed. If the problem persists, download the free version of Malwarebytes from http://www.malwarebytes.org/products/malwarebytes_free and run a full system scan to eliminate malware as the culprit as is often the case.