I cant see myself either, but students maybe pranking you with a greenscreen. If not, to me it looks like an issue with the applet, you could contact the website administrator from your university or wherever you teach. Did this help in anyway maybe a little bit ?

You can try to disable hardware acceleration in Firefox. *Options/Preferences -> General: Performance<br>remove checkmark: [ ] "Use recommended performance settings"<br>remove checkmark: [ ] "Use hardware acceleration when available" *https://support.mozilla.org/en-US/kb/performance-settings Close and restart Firefox after modifying the setting to make changes effective. *https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems You can check if there is an update for your graphics display driver and check for hardware acceleration related issues. *https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration ----- You can also check the Web Console for related messages. *"3-bar" menu button or Tools -> Web Developer -> Web Developer Tools *https://developer.mozilla.org/en-US/docs/Tools/Web_Console