Previously, Firefox was showing an error page saying “The address wasn’t understood” when a web page attempted to navigate to an unknown protocol using
<meta http-equiv="refresh"> or similar method. This behaviour had caused an user experience issue on various sites that tried to open their own external application.
Starting with Firefox 76, such page navigations/redirects will be ignored like Google Chrome, and Web Console will log a warning: “Prevented navigation to … due to an unknown protocol.”
If you need to launch an external application or simply check if an external application is installed, you can instead use either
window.open() or an