Hi Ed, Firefox's popup blocker blocks popup windows. I suspect what you are seeing in YouTube is an overlay in the page itself, or a notification, rather than a completely separate browser window. However, you are the one seeing it, so you would be in the best position to determine that.

Also, sites are allowed to show popup windows in response to a user action such as clicking a button, since that is very often useful. So popup blocking is really directed toward new windows that open automatically rather than in response to a user action.

Now back to what you are seeing: could you describe the message in more detail or share a screenshot of it? Please cut out or blur any sensitive content in an image before attaching it to your reply.

Here's a screenshot.

I tried to post a screenshot here, but all it did was "uploading file", but never completed. File too large (.bmp)?

Hi Ed, I don't know whether the site can convert a BMP image to PNG. I know it can convert a JPEG. If you are saving through Windows Paint, it can "Save As" to either PNG or JPG. (Other programs, I don't know.)

OK, now it is "Uploading Popup.jpeg". Do I have to wait for it to finish uploading?

This is confusing. I did an "Edit this question", and got the image uploaded, but had to go back to this window to make comments. Anyway, here is the uploaded image.

Hi Ed, I've seen that ad, too. This is an overlay in the page itself, and not a separate window.

Have you ever considered using an ad blocker? They can cause problems on some sites by blocking too much, but you may find them helpful for dealing with repeated annoyances because they often have a feature to hide an element of the page that you point them too, even if they aren't detected as ads. If you want to try one, here's a popular one to consider:

https://addons.mozilla.org/firefox/addon/ublock-origin/