They may be push notifications from the sites https://support.mozilla.org/en-US/kb/push-notifications-firefox I moved the thread from Firefox for iOS (for iPhone/iPad) to Firefox section as you have '''macOS''' and nobody on this support forum does direct support by phone, chat or email.

You can check for recently installed suspicious or unknown extensions. *https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware Start Firefox in <b>Troubleshoot Mode</b> to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if <b>hardware acceleration</b> or if <b>userChrome.css/userContent.css</b> is causing the problem. *switch to the Default System theme: "3-bar" menu button or Tools -> Add-ons -> Themes *do NOT click the "Refresh Firefox" button on the Troubleshoot Mode start window *https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode *https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems