For the past few versions, some variants of IE Tab have caused this problem. If you have a variant that has not been updated in the past few months, you may need to remove that one and install a new one. Or if you hardly ever use IE Tab, you could switch to an add-on like Open in IE that sends the page to Internet Explorer (outside of Firefox). To review/disable/remove extensions, use the Add-ons page. Either: * Ctrl+Shift+a * "3-bar" menu button (or Tools menu) > Add-ons In the left column, click Extensions. Then you can find IE Tab on the right side. If you want to try Open in IE, you can install it from here: https://addons.mozilla.org/firefox/addon/open-in-ie/ ''Note: This problem started in Firefox 40, and a couple other extensions were also implicated but most of them probably have been updated by now.''