I don't know why you can't switch tabs when that dialog is displayed, but you should be able to cancel it. For attack pages that do an immediate reload to keep that dialog displayed, you can press the Esc key numerous times to break the cycle. This is more effective than trying to click Cancel there and X in the address bar before the dialog displays again. (Unless you have superhuman mouse skillz.)

I tried your suggestion with the Esc key, but that didn't break the cycle at all. The only thing helping is quitting the whole browser...

Sorry, it worked on Windows the last time I was able to test. It's very possible the scripts they use have changed. Unfortunately, the URLs are encoded to prevent other people from using them, so it's hard to enlist others to investigate.

Do you recall what site this popped up from?

At the moment you can try to download almost any of the episodes on http://gogoanime.io . Click on one of the covers on the main page and then onto the download button on the upper left corner of the playing episode. Try that once or twice and it should popup ;-)

I don't seem to get anything interesting from the Download button. When I click the player to try to pause it, there's a popup/popunder each time, most commonly "update your Flash player." (What an annoying site.) I didn't find the auth one.

You're right. On Windows this site never comes up. So maybe they're targeting Mac users specifically ??

I was testing on a Mac, but it's possibly a pathway to the hack was shut down by now and a new one will pop up tomorrow...

Okay. Then there's nothing we really can do about it ;-)

It happens with a lot of small pop-ups it takes Control and becomes the active tab/window and will not relinquish control until it has been dealt with.

This is standard on a lot of sites and has been used for years. As to the coding of that I have no idea.

It is more obvious when it is something you do not want rather than something you have asked for. If you try that with something you have asked for you will notice that it is more common.

It has frustrated myself also when I want the window shrunk so I can get to the info behind it

Only way I know how to defeat with it in windows is to click the Show Desktop Button then everything goes down.........then pick what I need up.

No help but some additional info on that and if you could as showing version 57.0.1 please update to 57.0.2 if not already : https://support.mozilla.org/en-US/kb/update-firefox-latest-version

What worked for me is to click the cancel button with one hand and immediately press Ctrl+W with the other hand. That brought up a dialog to leave the page.

Thanks to all for the informations. the last tip did the trick for me ;-). I couldn't find a version 57.02 for Mac, seems to be Windows only.