Please explain when/how this happens.

When I open most websites, I havent been able to find out why some do but it doesnt always popup but once it does closing the popup and reloading the page usually pops up again.

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or userChrome.css is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Had another look this morning, didn't change anything, cant get it to popup again.

Ended up being the 'Easy Video Downloader' add on. Disabled and ok

That was very good work. Well Done.