Pop under windows are basically pop up windows, so Firefox should treat them the same way. Generally speaking, Firefox allows popup windows that are generated in response to certain user actions, and blocks those that are not in response to those actions. This allows sites to open windows when you click a link, for example, which may be bad behavior, but isn't the kind of severe abuse the feature was meant to prevent. So you might need an add-on to target pop up/under windows that Firefox treats as legit.

Maybe an option to disallow any new windows being opened for a specific web site would control the issue.

Hi mace2, the built-in popup control feature has a site-by-site permission setting, but as noted above, a lot of exceptions. There isn't a built-in way to have different exception lists for different sites.

I used to know of an add-on that could exert pinpoint control, but it stopped working a while back... Possibly a project for someone to work on?

You can give further feedback/encouragement on any of these sites:

• Input page: https://qsurvey.mozilla.com/s3/FirefoxInput/
• Add-ons discussion site: https://discourse.mozilla-community.org/c/add-ons
• Twitter: https://twitter.com/firefox
• Facebook: https://www.facebook.com/Firefox
• Reddit: https://www.reddit.com/r/firefox/