User Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:40.0) Gecko/20100101 Firefox/40.0 Build ID: 20150812163655 Steps to reproduce: I'm used Firfox as usual on Windows 10. After the update to version 40.0.2 the close button in the right corner doesn't work anymore. Actual results: When I hover the close button, the color changes to red. When i click on it, the color changes to brighter red, but nothing happens. Expected results: Firefox should close.