It's almost certainly one of your add-ons that's causing the issue. One is probably loading a popup utility onto the page, but it flashes for a split second before it's able to hide it. Not necessarily anything malicious, but it can be common for certain add-ons to do this depending on what functionality you need.

The easiest way to test is to try running Firefox in Safe Mode. This will temporarily disable all of your add-ons.

If the issue doesn't happen in Safe Mode (which I assume will be the case), then it's what I've described above.

Hope this helps.

The issue is with the website itself. I get the same behaviour, but only on the specific website in your video. It looks like the website is creating a popup, but then it hides it.

Thanks for the quick answer. I thought it was browser wide but only visible on some pages. Thanks again