This is caused by software from Trusteer Rapport. The software is almost certainly not essential, and removing it should not affect any existing Firewall and AntiVirus software you have. Please temporarily disable or remove that software. This is a known issue and mentioned in the Firefox45 [https://www.mozilla.org/firefox/45.0/releasenotes/ Release Notes]. Firefox developers are in contact with Trusteer. We understand Trusteer have a fix and that is being tested and rolled out, but I am not yet aware of when the updates will be made available to end users (YOU !). ------- P.S You may in fact find out before some of us do, so please confirm that Firefox is ok with Trusteer's Software removed, and also let us know when a suitable working upgrade to the software is available.