Previously, Firefox was raising the
NS_ERROR_UNKNOWN_PROTOCOL exception when attempting to load an unknown external protocol with an image, frame or whatever. The following HTML code snippet posted to the bug shows an alert dialog depending on whether a specific browser extension is installed.
<img src="sacore:green.gif" onerror="alert('McAfee Site Advisor not Installed.')" onload="alert('McAfee Site Advisor Installed!')">
Given that this could be a fingerprinting vector and other browsers don’t report an error, Firefox 64 and later will no longer raise an exception to better protect user privacy.