What is it showing? Is it a message, add or what?

Go to the Mozilla Add-ons Web Page {web link} (There’s a lot of good stuff here) and search for a good add / pop-up blocker.

They're not adds, they're little notifications in the upper right corner, from different programs like Facebook, or the Apple store about updates. It's not just Facebook, it's been a couple different websites. I'll try to catch it with a screen grab.

You can inspect and manage permissions for the domain in the currently selected tab via these steps:

• Click the "Site Identity Button" (globe/padlock) on the location/address bar
• Click "More Information" to open "Tools > Page Info" with the Security tab selected
• Go to the Permissions tab (Tools > Page Info > Permissions) to check the permissions for the domain in the currently selected tab

This includes the permission to show notifications. I don't know if what you see falls under such a notification.

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes