I suspect they are notifications, but just to give a complete answer:

The solution depend on which of the various types of popups they are:

(1) Popup window. This window has a limited set of bars (mainly just a non-editable address bar) and can be moved outside of the tab/window you were viewing.

See: Pop-up blocker settings, exceptions and troubleshooting

(2) Notification. This small panel typically opens consistently in the same area of the screen, whether coming from Firefox or from other programs. You may be need to remove site permissions to send notifications.

See: Web Push notifications in Firefox (check the "How do I revoke Web Push permissions for a specific site?" section)

(3) In page overlay. Some pages have "follow me" boxes for video ads, or will block their content with an overlay as you scroll down.

Firefox does not have a built-in feature to edit these annoyances out of pages. You could consider an extension like uBlock Origin. https://addons.mozilla.org/firefox/addon/ublock-origin/

(4) Extension panel. Extensions (a type of add-on) can create floating windows with no address bar. If you have a problem with these, you may need to clean up your add-ons.

You can view, disable, and often remove unwanted or unknown extensions on the Add-ons page. Either:

• click the Extensions button (puzzle piece) on the toolbar > Manage Extensions
• Command+Shift+a (Windows: Ctrl+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons
• type or paste about:addons in the address bar and press Enter/Return

In the left column of the Add-ons page, click Extensions. On the right side, find the "Manage Your Extensions" heading.

If there is at least one extension before the next heading -- "Recommended Extensions" -- please continue:

Then cast a critical eye over the list below that heading. Any extensions Firefox installs for built-in features are hidden from this page, so everything listed here is your choice (and your responsibility) to manage. Anything suspicious or that you just do not remember installing or why? If in doubt, disable (or remove). For your privacy and security, don't let mystery programs linger here.

Any improvement?

Note: Sometimes extensions can frustrate access to this page. In that case, first restart Firefox in its Troubleshot Mode and then try again. See: Use Troubleshoot Mode in Firefox -- Open, not Refresh.

I would say these are very likely to be web push notifications from sites you have allowed to send you notifications. Instead of useful information it way be junk, spam, fake virus/trojan alerts etc. You can revoke these sites from sending you notifications as the kb link below shows

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