User Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/67.0.3396.87 Safari/537.36 Steps to reproduce: A popup-ad opened and requested user credentials. Actual results: I wasn't able to close the window. When clicking cancel the credential window opened again immediately preventing the ad to be closed. I had to kill the firefox process and lost my other opened tabs. This can be harmful as the ad is very confusing, going to fullscreen when clicking somwhere on the window, trying to enable notifications and asking to install a plugin all at the same time. Something similar to this could trick users into installing harmful software. Expected results: I should have been able to close the window without killing the firefox process. The window for user credentials should not block the UI of the underlying window (at least not the close button).

Link to the ad (be careful when following this): http://foxmacff.cool/3/click/813021/?csum=bcMWXPe5xGog0_h0YdzA7ljk9NP9yhrWcABFduiBz8Ii2ZwGr6XbTOqkRpe9kYDxzzovpnMDgNFS7CPglbVZ1g%2C%2C&_subid=vlok6g1ajh1qqtnpncu&_token=uuid_vlok6g1ajh1qqtnpncu_vlok6g1ajh1qqtnpncu5b8151c12e9fc6.72570941 It also has some weird behaviour, like setting a custom cursor image so that it is no longer clear where your cursor is, so the user doesn't know what he actually clicks on. I have never seen such an obstrusive ad, but at least it highlights some security flaws.

Be aware that opening the html files without precaution might cause harm to your device as this seems to be a malicious popup ad.