Are these small popups on the bottom right corner of Firefox windows? If so then these may just be web push notifications from sites you have allowed to send you notifications from. Instead of useful information some may instead send you junk, spam, fake virus/trojan alerts. Your can revoke these sites as you can see by second link below.

https://support.mozilla.org/en-US/kb/push-notifications-firefox https://support.mozilla.org/en-US/kb/push-notifications-firefox#w_how-do-i-revoke-web-push-permissions-for-a-specific-site

The popups show up anywhere on the screen. If I've been in suspend for, say, overnight, there are dozens of them covering the screen. The links you show seem like useful information, but I'm reluctant to click on one of the popups and otherwise how do I determine where they're coming from?

Did you try with Firefox in Troubleshoot Mode? https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode#search

Does it happen under a different user account?

Troubleshoot mode make no difference. In 'Settings - Notification Permissions' I see the list there, but how to I know which one? How do I find out which/what is sending the popup?

I deleted a bunch of them and it solved the problem. But I still think it would be a good idea if you could figure out which site was sending any given popup.