This started failing a few months ago. Not sure if was from an update of Firefox or bad cookie. The pop-up to sign-in with browser launches a window comes up with another small window pops up and they both disappear. It is throwing an exception. I tried uninstalling and reinstalling and the same happens and couldn't complete the setup process. I did however discover a workaround to this. Go to a new Firefox browser window and sign into your Capital One account Then go back to the window you want to use a virtual account number for, select the Eno extension and it properly works. After I did this once, after logging out of Capital One. I tried again selecting the Eno externs and the sign-in with browser launches a pop-up window and allows me to sign-in